If they didn't already, now they also have to worry about the Milwaukee Brewers.
The Brewers gave the Cubs a good challenge in 2017, staying in first place as late as July 25 and finishing just six games back at 86-76. Not bad for a team that had embarked on what was supposed to be a rebuilding year with just a $63 million payroll, the smallest in Major League Baseball.
And now they have two shiny new toys to play with.
The Brewers' first big move Thursday was a blockbuster trade with the Miami Marlins for star outfielder Christian Yelich. The Athletic's Ken Rosenthal first reported the deal, and the final terms came from the team itself:
Before the social media chatter about that swap had a chance to die down, the Brewers struck again by signing Lorenzo Cain, yet another star outfielder. ESPN's Jerry Crasnick had the details:
This is quite the splash, yet to call it unexpected is a stretch.
After all, the Brewers have consistently been linked to all sorts of big names throughout the offseason. Team owner Mark Attanasio practically promised that would be the case, as he was looking forward to putting the club's resources to good use before last season was even over.
"It helps level the playing field versus teams that have more resources," Attanasio said in late September, according to Tom Haudricourt of the Milwaukee Journal Sentinel. "We're in a fortunate position so we can maybe 'punch a little bit above our weight.'"
Among those resources is spending power that supported $100 million payrolls as recently as 2014 and 2015. And, a farm system that checked in at No. 7 in Bleacher Report's latest rankings and landed six prospects in Baseball America's top 100 for 2018.
The latter number is down to four.
In Lewis Brinson and Monte Harrison, the Brewers are giving up the No. 18 and No. 75 prospects in baseball, respectively. Isan Diaz is no mere throw-in, as he was a consensus top-100 prospect going into last season. Jordan Yamamoto may not be either, as he's coming off a 2.51 ERA in 22 appearances for High-A Carolina.
But the Brewers can rest easy knowing that Brinson and Harrison were part of a deep collection of young, talented outfielders that still includes Domingo Santana, Keon Broxton and Brett Phillips.
They can rest even easier knowing that those two helped bring back a player who's young, controllable, cheap and, oh yeah, excellent.
Yelich may not be a household name, but he's a .290 career hitter with an .800 career OPS who can also run and play good defense. He's averaged 4.1 wins above replacement since 2014, which is higher than any Brewers position player climbed in 2017.
Even better: Yelich is only 26 and signed to a deal that stretches as far as 2022, where it would max out at a modest $15 million.
For his part, Cain is deserving of the biggest contract of the offseason. He was a big component of the Kansas City Royals' rise to excellence in recent years, averaging 5.1 WAR from 2014 through 2017. And although he's on the older side at 31, his legs, bat and glove are in good shape.
Rather than whether these are good moves on their own, a better question is whether their timing is right.
Although the Brewers' 2017 season was a pleasant surprise, in retrospect it doesn't look like an obvious springboard to bigger and better things. They were supposed to be bad and became merely good despite having a whole bunch of things go right. Many of their weaknesses—namely: second base, catcher, center field and starting pitching depth—still loom with the 2018 season getting closer.
At the start of the day Thursday, it wasn't surprising to see FanGraphs projected the Brewers to win only 73 games. That was 19 fewer than the Cubs, 15 fewer than the Cardinals and even five fewer than a Pittsburgh Pirates team that doesn't even have Gerrit Cole and Andrew McCutchen anymore.
Milwaukee wasn't without reasons to feel bullish about, however.
There's a real possibility Eric Thames (1.4 WAR), Travis Shaw (1.8 WAR) and Domingo Santana (1.5 WAR) will pick up where they left off in 2017 and outperform their modest projections. It's also possible former top prospect Orlando Arcia (1.0 WAR) will blossom as a star shortstop after getting his feet wet last season.
In light of the recent report, per MLB.com's Adam McCalvy, that right-hander Jimmy Nelson is ahead of schedule in his return from shoulder surgery, the Brewers can also look forward to reuniting the trio of Nelson, Chase Anderson and Zach Davies that anchored the club's starting rotation in 2017.
These were solid excuses to imagine a few extra victories on top of Milwaukee's 73-win projection. Bringing Yelich and Cain into the fold is worth still a few more. So at this juncture, it's within reason to look at the Brewers and see a National League wild-card contender.
And they may not be done.
Earlier this week, a report from Yahoo Japan (h/t Mark Polishuk of MLB Trade Rumors) claimed the Brewers had made a contract offer to ace right-hander Yu Darvish. He's only the latest big-name starter they've been linked to. Indeed, Jon Morosi of MLB.com linked them to Jake Arrieta and Lance Lynn at the winter's outset.
Even after trading for Yelich and signing Cain, the Brewers may yet be able to be afford one of these guys. Failing that, they have an outfield surplus from which they could pull to find a top-tier starter on the trade market.
Regardless of how they do so, adding an extra starter could give the Brewers enough pieces to challenge the Cubs and Cardinals for the NL Central title in 2018.
If not, there wouldn't be signs pointing them back to the drawing board. They're still going to have a fairly deep farm system under them as well as some future spending flexibility with Braun coming off their books as soon as 2020. They could keep adding in subsequent years, either from within or without.
If there's a rival that ought to be worried about this, it's the Cubs.
There's little doubt Chicago will be one of the National League's heavyweights once again in 2018. But the cracks that showed last year haven't fully mended. The Cubs need bounce-back or breakout seasons from quite a few of their lineup regulars. They also haven't found anyone to fill Arrieta's shoes in their starting rotation.
Trouble is, the Cubs' resources aren't what they once were. Their farm system has gone from being the best of the bunch to one of the worst in short order. Meanwhile, their payroll is weighed down by multiyear deals and homegrown players who are entering their arbitration seasons.
So, when Chicago won 103 games and captured a very-long-awaited World Series title in 2016? It's fair to wonder if that was its peak and if it'll be ripe for toppling in coming years.
The Brewers could have done it by playing the long game and using in-house promotions to methodically construct a superteam over several years. Instead, it seems they've looked at the layout on the board and come to a perfectly logical conclusion:
Why wait when the playoffs are right there, right now?