Like the team, Lester clearly possesses supreme talent, as he was the 12th-best pitcher in baseball from 2009-11. However, neither Lester nor the Sox parlayed that talent into results, putting up sub-par performances in recent years.
After slumping from May through July, it felt like Red Sox Nation was ready to give up on Lester. But since the All-Star break, a funny thing has happened: Lester's season has turned on a dime, and he has been one of the best starters in the second half. Considering Clay Buchholz's long-term absence has robbed the Red Sox of a true ace, Lester's timing is extremely fortuitous.
However, the lefty also exhibited similar form in April, only to see his performance plummet to sub-replacement levels. So while Red Sox fans are undoubtedly ecstatic at this bounce-back stretch, skepticism at Lester's sustainability would be perfectly understandable.
Given his inconsistencies, it's virtually impossible to definitively conclude whether or not Lester will be an asset for the Red Sox down the stretch. But examining the evidence already presented, here are the best reasons to both believe and doubt the southpaw's second-half form.
Reasons to Believe
Much of Lester's success stems from a completely renovated approach, specifically towards his pitch utilization. From 2009-11, Lester had one of the best cutters in baseball, and it served as his out pitch. But this year, his go-to pitch has largely abandoned him, with a value of about eight runs below average.
Since the All-Star Break, Lester has significantly curtailed the cutter's use after a stubborn adherence to its diminishing effectiveness the past two seasons. Hitters have yet to adjust to that variety, and as he told MassLive.com's Evan Drellich, Lester believes that has kept them off balance:
Obviously I think if you go and talk to anybody that's going to face me, I think that's always in the back of their mind as far as the pitch I'm going to try to get you out with. So I think that if you're able to maybe show it a little earlier and show some pitches that you don't normally throw later in the count, you'll get some mis-hits or some swings and misses or whatever. It's a game of adjustments. You always have to constantly make adjustments. We're doing that right now.
Indeed, Lester has thrown his cutter less than 20 percent of the time in five of seven starts since the break. Lester hasn't necessarily replaced the cutter's usage with one pitch; his fastball has consistently been his best pitch, perhaps more effective with the increased usage of off-speed pitches. Notice how much poorer contact he is inducing with the pitch now:
Lester's success has actually stemmed not from a return to his high-strikeout heyday, but rather a complete departure from it. Compare his pitches before and after the All-Star Break—hitters are swinging at significantly more pitches and making more contact:
That means Lester is pounding the strike zone more often, a fact belied by his walk rate decreasing from 3.22 to 2.31 in the second half. The strikeouts have been inconsistent from game to game, though his overall rate is about the same in as the first half, but that is simply a byproduct of Lester pitching to more contact.
By no means is Lester suddenly a junk-ball pitcher. Most importantly, Lester is showing greater in-game awareness of the opposition, rather than simply trying to power through on his natural ability. The 29-year-old lefty is pitching with more intelligence than ever, a critical and likely sustainable adjustment as he continues to age.
Reasons to Doubt
Though pitch variety has flummoxed opposing hitters thus far, the numbers might start to tilt the other way once teams start adjusting to Lester's new approach. In fact, Lester's peripherals have essentially mirrored each other—despite the cavernous ERA difference, his 3.97 first-half xFIP is almost identical to his 3.98 second-half mark:
What do you expect from Jon Lester the rest of the season?
Lester's second-half BABIP is down to .281, but that is not necessarily unsustainable if he truly has changed his approach. What's more concerning is Lester's skyrocketing fly-ball rate and shrinking ground-ball rate. The sample size is still relatively small, but since the break, Lester's 41.8-percent fly-ball rate is 17th-highest among all starters, and his 36.2-percent ground-ball rate is the 18th-lowest.
So how has he survived? The answer might be as simple as location. Five of Lester's seven starts since the break have come on the road, including pitcher-friendly ones like Kauffman Stadium, AT&T Park and Dodger Stadium. Lester's one dud, a 4.1 inning, six-run outing against the Diamondbacks, came at Fenway.
Lester's home-road splits are not particularly divergent this year, but it's no secret that extreme fly-ball pitchers are prone to extra-base hits at Fenway. Obviously that does not mean Lester is doomed every time he takes the mound wearing the home whites, but his 5.1 percent HR/FB ratio is one of the lowest since the break, perhaps signaling that some future flyballs will find their way into the Monster seats.
And even if Lester's fly-ball luck does continue, there's the matter of linedrives and pop-ups. Those two represent the extremes of contact, as one is quite likely to result in a hit, while the other is an out nearly 100 percent of the time. Unfortunately, it appears Lester is due for some regression in that sense as well:
It's extremely hard to continue throwing as many strikes as he has without giving up worse contact, and it's not encouraging that his pitches out of the zone have not been inducing very many pop-ups. Even if you are part of the stubborn minority that still resists advanced stats, watch how three fortuitous double plays against the Dodgers saved Lester from some jams.
This does not mean Lester will turn back into a pumpkin by September. But regression towards the mean is practically inevitable, and pitching to contact is often a dangerous exercise, particularly for someone who relied so heavily on strikeouts.
The Big Picture
Before Red Sox Nation works itself into a tizzy over the last section, Lester's post-break 3.07 FIP is one of the 25 best in the league. Even if it does signal a little home run luck, regression towards that number would not be catastrophic at all.
The real question is this: When Lester does experience some statistical regression, will his new approach be enough to compensate and foster long-term success?
The sample size is too small to make any definitive conclusions, and it would certainly be nice to see more starts at Fenway before making any sweeping statements. It appears his next two starts will come at home against the Orioles and Tigers, two of the top five offenses in baseball, so Sox fans should have better answers two weeks from now.
Ultimately, if the Red Sox are going to make a World Series run, there is not a large margin of error for Lester. Clay Buchholz is a total wild card, while the likes of Jake Peavy and John Lackey are solid No. 2 and No. 3 starters, respectively. As mind-boggling as it might have seemed in June, Jon Lester might once again be the ace of the Boston rotation. Whether Lester is a true ace or simply one by default will likely determine the fate of the Red Sox's postseason and World Series hopes.
*All stats courtesy FanGraphs.