Ranking the 10 Biggest MLB Series in September That Will Decide 2025 Playoff Races
September has arrived, which means October baseball—a.k.a. Major League Baseball's postseason—is just around the corner.
It's so close and yet so far away, with many critical series left on the docket to determine what the bracket will look like when the playoffs begin.
Though we have a pretty good idea at this point who the 12 playoff teams will be, nothing has been decided yet. Even Milwaukee at 15.5 games clear of Cincinnati as the NL's current "No. 7 seed" is still at least six days away from clinching a postseason berth.
There are plenty of series in September that could be important, but 10 in particular look destined to be crucial in framing the final picture, which we have ranked here.
Honorable Mentions (in chronological order): Mets at Tigers (Sept. 1-3), Phillies at Brewers (Sept. 1-4), Yankees at Astros (Sept. 2-4), Mets at Reds (Sept. 5-7), Astros at Rangers (Sept. 5-7), Tigers at Yankees (Sept. 9-11), Royals at Phillies (Sept. 12-14), Rangers at Astros (Sept. 15-17), Blue Jays at Royals (Sept. 19-21), Brewers at Padres (Sept. 22-24), Mets at Cubs (Sept. 23-25), Tigers at Red Sox (Sept. 26-28)
10. Houston Astros vs. Toronto Blue Jays
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When and Where: Sept. 9-11 in Toronto
What's at Stake: AL East and AL West division races; first-round bye
Toronto just wrapped up a nine-game stretch against Pittsburgh, Miami, and Minnesota, but managed only a 5-4 record. Now, the Blue Jays face a tough 28-game finish, starting with a series loss to Milwaukee.
Even the so-called "easy" games against Baltimore and Tampa Bay will be challenging, and every remaining opponent is fighting for a playoff spot.
And the Astros sure did have Toronto's number earlier this season, sweeping the Blue Jays in Houston by a cumulative score of 15-2 in April.
A repeat of that would be disastrous for the Jays, in light of the AL East roundtable coming up shortly on this list. It would also be a dagger to Seattle's hopes and dreams of winning the AL West for what would be the first time since 2001, as the Astros otherwise have a pretty manageable remaining schedule.
If Toronto returns the favor, though, say hello to a potential flip in the AL West, as the Mariners will be hosting the Cardinals and Angels while this series is going down.
Even if both teams manage to hang onto their current division leads regardless of what happens in this series, though, it figures to play a key role in determining the order of the top three seeds in the American League; the loser of this series maybe destined for the No. 3 seed and a death-defying trip to the Wild Card Round.
9. New York Mets vs. San Diego Padres
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When and Where: Sept. 16-18 in New York
What's at Stake: Wild-card seeding; possible NL East and West races
Though the quartet of Cincinnati, San Francisco, Arizona and St. Louis is nowhere near mathematically out of the running for an NL wild-card spot, it does feel like we know the six teams that will appear in that half of the bracket in a month's time.
But a showdown between the current No. 5 (SDP) and No. 6 (NYM) seeds surely could have some major postseason implications.
If the Padres sweep the Mets, maybe it helps them overtake the Cubs for home-field advantage in the Wild Card Round, or perhaps it even gives them enough of a boost to jump back ahead of the Dodgers in the NL West race for a possible first-round bye. Meanwhile, losing all three games could leave the Mets prone to possibly getting caught from behind by the Reds or Giants.
If New York takes care of business at home, though, there's a decent chance the Mets leapfrog the Padres for the No. 5 seed and maybe even track down the Phillies for the NL East crown.
However, since this series comes after the Mets open September with 10 straight road games against Detroit, Cincinnati and Philadelphia, goodness only knows where they'll be at heading into this matchup with the Padres.
8. Kansas City Royals vs. Seattle Mariners
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When and Where: Sept. 16-18 in Kansas City
What's at Stake: Final spot in the American League field
With all due respect to the Texas Rangers—who do have six games left against Houston but seem to be losing yet another star player to the season-ending IL on a daily basis as of late—the team with the best chance of crashing the postseason party is probably the Kansas City Royals.
If they're going to make that happen, knocking out the current No. 6 seed is surely the path of least resistance.
Lo and behold, they have a gigantic opportunity at home against said No. 6 seed in a little over two weeks' time.
Will Cole Ragans be back on the mound by then?
And will they give Jac Caglianone another chance to save their season, in light of the way he has been obliterating baseballs again at Triple-A over the past three weeks?
For lack of a better term, the Royals are the ultimate wild card in this wild-card race, liable to close what is already a minimal gap over the next seven days at home against the Angels and Twins while the M's finish out their nine-game road trip with games against sub-.500-but-feisty Tampa Bay and Atlanta.
When these teams met earlier this season in Seattle, it couldn't have been much more even, splitting 2-2. Notably, that means if Kansas City takes two out of three (or all three), it would own the head-to-head tiebreaker, should it come down to that.
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7. New York Mets vs. Philadelphia Phillies
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When and Where: Sept. 8-11 in Philadelphia
What's at Stake: NL East crown, New York's spot in the postseason
Of the more than 20 series remaining between the 12 teams presently in the projected postseason field, this is the lone four-game set of the bunch. Thus, all other things being equal, the stakes are 33 percent higher here, with the possibility of a four-game sweep that completely flips the script.
With a six-game lead in the NL East, though, it almost feels like the Phillies just need to avoid being on the wrong end of such a sweep in order to ultimately win this division.
If they are, instead, the ones delivering a four-game sweep—like they did at home against New York two Septembers ago—that would be huge in their quest for a first-round bye, while also potentially devastating to New York's plans of staying comfortably ahead of Cincinnati and Co. in the wild-card race.
And it's looking like the Mets' fate in this series will rest squarely upon the shoulders of some brand new pitchers. 23-year-old Nolan McLean has made three starts thus far in his MLB career, while 22-year-old Jonah Tong just made his debut on Friday. Nevertheless, the former is lined up to start in Game 2 of this series; the latter in Game 4.
Kind of wild that a team with a (luxury tax included) payroll of nearly $425 million is about to be mighty dependent upon a pair of rookies making a combined, oh, $0.255 million this season.
Nos. 6-4: AL East's Remaining Round Robin
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When and Where: Toronto at New York (Sept. 5-7), New York at Boston (Sept. 12-14), Boston at Toronto (Sept. 23-25)
What's at Stake: Division crown, first-round bye, wild-card seeding
Of this trio of huge AL East series, the most important is the one on tap first.
If Toronto waltzes into New York this coming weekend and improves from 7-3 to 10-3 against the Yankees this season, that's probably curtains on the AL East race, and Toronto can instead set its sights on home-field advantage through the ALCS.
But even a Blue Jays sweep in the Bronx would make the subsequent New York-Boston series more intriguing as a possible "winner gets to host the No. 4 vs. No. 5 wild-card series" extravaganza between no-love-lost rivals.
If, however, the Yankees take that series, game on. They've already turned what was a 6.5-game deficit barely a week ago into a three-game nail-biter by trouncing both the Nationals and White Sox. And with a season-ending stretch of 13 games against the Twins (three), White Sox (three) and Orioles (seven), you almost have to assume New York is going to finish things out on a high note.
Either way, though, Boston—at a manageable 3.5 games behind the Blue Jays and just half a game behind the Yankees—holds a lot of its destiny in its own hands, even after a back-breaking series loss at home against Pittsburgh this past weekend.
In addition to the two series here, the Red Sox have six games left against the A's, as well as three each against Cleveland, Arizona and Tampa Bay. And if they win the aforementioned series in Toronto, there's a decent chance Boston's season-ending series at home against Detroit becomes a situation of hosting a team that has locked up the No. 1 seed and is just looking to avoid injury.
It's likely all three of these teams will make the postseason, though. It's just a question of who gets seeded where.
3. Los Angeles Dodgers vs. Philadelphia Phillies
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When and Where: Sept. 15-17 in Los Angeles
What's at Stake: First-round bye, NL East and NL West division crowns
It's getting close to a foregone conclusion that the Milwaukee Brewers will secure the No. 1 seed in the National League, but the No. 2 seed and the coveted first-round bye that comes with it are still very much up for grabs.
As things stand, the Phillies hold a slight edge over the Dodgers in that race. It's hardly just those two teams, though. San Diego is close on L.A.'s tail, and though the Mets have by far the most difficult remaining schedule of the NL's projected playoff teams, they aren't out of the running, either.
Most likely, though, it's going to boil down to either the Phillies or the Dodgers, in which case, this head-to-head showdown looms all sorts of large.
If the respective rotations hold to form, it's going to be Cristopher Sánchez vs. Emmet Sheehan, Jesús Luzardo vs. Clayton Kershaw and Taijuan Walker vs. Shohei Ohtani. Not exactly an NLCS preview, devoid of Yoshinobu Yamamoto, Blake Snell, Tyler Glasnow, Aaron Nola and Ranger Suárez, but some good matchups all the same.
Of course, the biggest matchup of all isn't at pitcher, but rather at designated hitter, with Ohtani and Kyle Schwarber squaring off with the NL MVP trophy possibly hanging in the balance.
When these teams met in early April, Ohtani and Schwarber went a combined 2-for-19 with no extra base hits. (And Ohtani wasn't pitching then.) But if either paces his team to a critical series victory, it could be the deciding factor in more than just the NL's No. 2 seed.
2. Houston Astros vs. Seattle Mariners
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When and Where: Sept. 19-21 in Houston
What's at Stake: AL West crown; loser's spot in the postseason
Very strong case for No. 1 right here, but we're reserving that spot for a different Mariners series with all of the "do-or-die" implications on the final weekend of the regular season.
As things stand, Seattle is two games behind Houston in the AL West, as well as four or fewer games ahead of each of Kansas City, Texas and Cleveland in the race for the AL's final wild-card spot.
Truly, every one of the Mariners' remaining series could have massive ramifications on the final playoff picture.
But this one is particularly large, with a chance to instantly claw all the way back into first place of a division where Houston has held at least a share of first place for 90 consecutive days.
Granted, a lot could change in the nearly three weeks left until this series begins.
However, considering Seattle opens September with 13 consecutive games against teams with losing records—Tampa Bay, Atlanta, St. Louis and the Angels—while Houston begins a nine-game gauntlet against the Yankees, Rangers and Blue Jays on Tuesday, it's hard to imagine the M's will have fallen hopelessly behind the Astros by the time this series begins.
If anything, the change is likely to be that the two find themselves in a dead heat with nine games left to be played, and this three-game set determining who ends up with the head-to-head tiebreaker, as the season series is currently tied at five wins apiece.
If that's the case, the loser of this series not only has a borderline insurmountable quest back to the division lead, but it also has to hope it doesn't also fall behind the Royals or Rangers and out of the playoff picture altogether.
It's a little far out to be projecting the starting pitching matchups, but with how things are lined up right now, there's a reasonable chance it ends up being Bryan Woo vs. Spencer Arrighetti, George Kirby vs. Hunter Brown and Logan Gilbert vs. Framber Valdez.
If those are the matchups and these teams are tied heading into it, it's going to be all of the edge-of-your-seat entertainment.
1. Los Angeles Dodgers vs. Seattle Mariners
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When and Where: Sept. 26-28 in Seattle
What's at Stake: West Division crowns; Seattle making the postseason at all; MVP races?; World Series preview?
For the most part, the final few days of the regular season are devoid of guaranteed theatre. With the exception of this colossal interleague showdown, the only series between teams currently either in the postseason picture or within three games of it is the Detroit-Boston series—which we've already mentioned may well be a situation where Detroit's primary goal is to not suffer any injuries, already locked into the AL's No. 1 seed.
But that just means all eyes will be on Dodgers-Mariners, with so much hanging in the balance.
Los Angeles will probably have clinched a playoff spot by this point, but will it have won the NL West? Or secured a first-round bye ahead of the NL East champ? If not, the Dodgers will be throwing everything they've got at the M's, hoping to earn a few days off during the Wild Card Round.
And pending the outcome of the aforementioned Houston-Seattle extravaganza, where will the Mariners be in the American League picture? AL West leaders looking for a first-round bye? Clinging for dear life to the No. 6 seed? Needing a sweep to catch Kansas City or Texas for that last spot in the field?
Also, what's Cal Raleigh's home run count at this point? If he enters the series at 61 and ends up getting to 63 via a dinger off Shohei Ohtani, could that be a case of the NL MVP serving up a history-making moment to what ends up being the AL MVP? (If Ohtani is even still ahead of Kyle Schwarber in his quest to become just the second player in MLB history with at least four MVPs?)
Or if both teams have already secured their spot in the playoff field, could this merely be a repeat of 2022, when Houston and Philadelphia squared off to end the regular season, a few weeks before waging war in the World Series?
No matter what, this series should be a fantastic springboard into October.
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