On Tuesday, for the second time inside of a week, two of the NL’s top starting pitchers in terms of ERA — the Cubs’ Jon Lester (2.10, third in the league) and the Dodgers’ Ross Stripling (1.99, which would rank second if he weren’t 4.2 innings short of qualifying) — will square off, this time in Los Angeles. On June 20, Lester got the upper hand, throwing seven shutout innings in a 4-0 win, the latest strong outing for the 34-year-old southpaw, who’s been on quite a roll lately.

Indeed, Lester has surrendered a mere two runs and 13 hits in his last four starts (27 innings), both via solo homers by Cardinals in a June 15 game that the Cubs won, 13-5. Only once in his past 10 starts has he allowed more than two runs (four in six innings versus the Pirates in a May 29 win), good for a 1.58 ERA over 62.2 innings. Depending upon the schedules of Max Scherzer (10-3, 2.09 ERA) and Jacob deGrom (5-3, 1.69 ERA) as well as the preferences of their respective teams, it’s not completely farfetched that NL All-Star manager Dave Roberts could give Lester (who’s a gaudy 9-2 to go with that ERA) the start on July 17 at Nationals Park, though you can imagine the pressure will be on the Nationals to make Scherzer available, health permitting.

Despite those superficially glossy stats, Lester is nowhere near the top of the NL pitching WAR leaderboard. His 0.9 WAR ranks just 26th in the NL, somehow behind the WARs of the likes of the Marlins’ Jose Urena (2-9, 4.40 ERA, 1.4 WAR), the Phillies’ Vince Velasquez (5-8, 4.69 ERA, 1.4 WAR), and the Mets’ Zack Wheeler (2-6, 4.85 ERA, 1.2 WAR), none of whom are likely to make the NL All-Star team, let alone get consideration for the start.

The disconnect for Lester is that his FIP (4.19) is almost exactly double his ERA, ranking 28th among the 43 pitchers with enough innings to qualify and 37th out of 59 with at least 60 innings; his 104 FIP- tells us that he’s actually 4% worse than league average on that front. The 2.09 runs per nine differential between his ERA and FIP isn’t just the majors’ largest this season, it’s the largest from an ERA qualifier since 1901. Even if you drop the innings threshold to 90 (Lester’s total), he’s just a whisker away from the lead:

I’ll forgo a deeper dive into those other pitchers, none of whom came close to sustaining their high-wire act over an ERA-qualified complement of innings, in favor of maintaining the focus on Lester and asking, “How’s he doing that?” For starters, his FIP is the product of middling peripherals. His 1.0 homers per nine, though solidly above his career mark (0.86 per nine) isn’t of much concern in a league where starters are serving up 1.18 per nine. However, he’s striking out just 7.0 per nine (down from 8.97 just a year ago), his lowest rate since 2008, and walking 3.1 per nine, his highest mark since 2011. On a per plate appearance basis, his 19.3% K rate is his lowest since 2012, and his drop from last year’s 23.6% is even more stark considering that NL starters as a group have climbed from 20.7% to 22.0%; that’s a swing of 5.6 percentage points relative to the league.

Lester’s sterling ERA owes a great deal to the Cubs’ defense, which leads the NL in Defensive Efficiency (.716). The team has been working extra hard behind him, helping Lester to a .231 BABIP, the league’s third-lowest mark, and a hefty 79 points below last year’s mark. It’s not because he’s giving up less hard contact, either — and, in fact, his batted-ball profile is rather out of character:

Jon Lester via Statcast, 2015-2018 Year GB% FB% GB/FB EV LA wOBA xwOBA Dif 2015 48.9% 29.3% 1.67 85.7 8.6 .289 .276 .013 2016 46.9% 32.8% 1.43 86.1 10.7 .264 .277 -.013 2017 46.2% 32.4% 1.42 85.4 9.3 .320 .308 .012 2018 38.6% 39.0% 0.99 89.2 12.0 .281 .355 -.074 SOURCE: Baseball Savant EV = average exit velocity, LA = average launch angle

Since coming over to the Cubs in the winter of 2014-2015, Lester has generally been in the middle of the pack or a bit higher as far as ground-ball rates go, at least among NL starters; last year, he was 16th out of 31 NL qualifiers, the year before that, 29th out of 70. This year, he’s 37th out of 43, producing more flies than grounders for the first time since his abbreviated 2007 season. And as his Statcast metrics show, he’s generally giving up harder contact, with an average exit velocity that’s up 3.8 mph from last year; it’s the majors’ 24th highest of the 106 pitchers with at least 200 batted balls. Combine that with a significantly higher average launch angle than before and you have a pitcher whose xwOBA is up 47 points over last year and 78 points over two years ago, ranking 91st among the 118 pitchers who have thrown at least 1000 pitches this year.

A quick look at the top of the leaderboard — Stripling (.237), Justin Verlander (.238), Chris Sale (.253), Scherzer (.254), deGrom (.258), Aaron Nola (.268), Noah Syndergaard (.274), Corey Kluber (.277), Trevor Bauer (.278), Gerrit Cole (.281) — confirms that the coolest kids in this stat are generally at the other end of the bus in terms of ERA, FIP, or WAR. Among those same 118 pitchers, only Arizona’s Matt Koch (-.090) and Oakland’s Sean Manaea (.076) have larger gaps between their wOBA and xwOBA.

So it would seem Lester’s gotten fairly lucky when hitters have made contact — they’re hitting it harder off him than in any of his Chicago seasons, but runs aren’t scoring. As to where those strikeouts and ground balls have gone, it starts with his dip in velocity (91.5 mph average for his four-seamer, just 0.2 mph below last year but 1.6 mph below 2016), fewer swings and misses overall (9.0%, down from 10.9% last year), and a changing pitch mix:

Lester’s four-seam fastball and cutter rates (47% and 22%, respectively) are about where they were in 2016, when he posted a 2.41 ERA, 3.40 FIP and finished second in the NL Cy Young voting while helping the Cubs to their long-awaited championship. He’s more or less shelved his sinker (4.4%, down from 12.3% last year) while throwing his changeup (11.1%) about twice as often as two years ago and more than at any time since 2013; his usage of the curve has slowly climbed, as well. His sinker produced — and still produces — a ground-ball rate just south of 70%, but this year, it’s been hit hard, and he’s kept it in his pocket, avoiding it entirely in each of the past two outings and throwing three or fewer in seven of his past 11 turns.

Meanwhile, although batters aren’t swinging and missing against his curve as often, particularly outside the zone, they’ve hit just .139/.184/.194 in the 38 PA that have ended with a hook, striking out 13 times and walking none (he hit two). That’s a 13 wRC+, similar to his 7 (yes, seven) from two years ago; it was 63 last year and is at 38 for his career. It’s just that, relative to years past, they’re hitting fewer ground balls with it (40.9%, compared to a 50.7% career rate). His changeup has been similarly stifling, as batters have hit .145/.175/.182 in the 57 PA that have ended with the pitch, striking out 11 times, walking twice and — as is also the case with the curve — failing to homer. His 5 wRC+ with the pitch is a Wile E. Coyote fall from last year’s 107 and is well below his career mark of 72 as well.

So Lester is getting exceptional results from his curve and changeup, two pitches that together account for 25% of his arsenal. Are they sustainable? At first glance, it almost seems possible, at least based upon his wOBAs and xWOBAs:

Jon Lester’s Curve and Changeup, 2015-2018 Year. Pitch wOBA xWOBA Dif 2015 Curve .219 .203 .016 2016 Curve .166 .165 .001 2017 Curve .250 .233 .017 2018 Curve .172 .238 -.066 2015 Change .247 .214 .033 2016 Change .281 .191 .090 2017 Change .315 .294 .021 2018 Change .158 .263 -.105 SOURCE: Baseball Savant

Right now, Lester’s wOBAs via both pitches are below .200; he finished in that general vicinity with the curve in 2016, though he was producing even less favorable contact (as indicated by xwOBA) then. He’s never done it with the changeup — though, again, he’s gotten his full-season xwOBA below .200 with the pitch. It’s asking too much for him to sustain his differentials, however, particularly when his average exit velocities are so much higher than they were in 2016. He’s currently at 87.5 mph on contact with the curve and 88.3 mph for the changeup, compared to 83.5 mph and 82.3 mph, respectively, in 2016. Simple math tells us that to shave 4 mph off of those, over roughly half a season, would require average exit velos 8 mph lower. Nuh-uh, Bubba, ain’t gonna happen.

So Lester is having a superficially strong season thanks to his particularly effective curve and changeup, and in spite of a trend towards harder contact. Unless he can curb that latter tendency and miss more bats, it seems highly likely he’ll regress, particularly given the historic ERA-to-FIP differential. But so long as he’s keeping runs off the board, the Cubs — whose new-look rotation featuring Yu Darvish (who’s likely to return from a triceps strain this weekend), Tyler Chatwood and Jose Quintana has otherwise been middling at best — will take it.