Chris Sale is back. Better than ever? Not quite. But Sale’s return from Tommy John surgery has gone about as well as the Red Sox ever could have hoped.

The left-hander has hit the ground running since he came off the injured list in August, posting a 2.53 ERA with 24 strikeouts and five walks in 21 1/3 innings through four starts -- all Red Sox wins.

Sale’s first start came roughly 17 months after he underwent his elbow procedure in March 2020 and almost two years to the day since he threw his last pitch for Boston.

The 32-year-old had soft matchups against the Orioles, Rangers and Twins his first three times out, but he took the mound against the American League East-leading Rays -- MLB’s No. 1 offense in terms of runs per game -- last Wednesday at Tropicana Field and held them to two runs in six innings, his longest start of the season.

Sale is lined up to face Tampa Bay again on Monday, this time at Fenway Park. With the Red Sox clinging to an AL Wild Card spot and Boston’s rotation sporting a collective 4.51 ERA since the All-Star break, every Sale start is close to a must-win game for the club at this point.

Can he continue to deliver? His four-seamer will be key.

It’s a small sample size, but the pitch has been a dominant offering so far. Among the 175 starting pitchers who have ended at least 40 plate appearances on four-seamers this season, Sale has the lowest xBA (.147) and the lowest xSLG (.247) allowed on the pitch.

Sale’s 34.6% whiff rate on four-seamers is also in the upper echelon -- only two starters (Joe Ross and Yu Darvish) have a better figure (min. 50 swings on four-seamers).

Sale started throwing a four-seamer as his primary fastball in 2017, but he didn’t crack the 30% whiff rate threshold with the pitch from ‘17-19.

Sale’s four-seamer whiff rate by year
2017: 27.3%
2018: 29.6%
2019: 28.5%
2021: 34.6%

Location has played a big part in Sale’s four-seamer success. In 2019, Sale threw 29.3% of his four-seamers in the heart of the plate. That number is down to 24% this year.

At the same time, the southpaw has been going up the ladder more often, challenging hitters in the upper half of the zone and higher. Sale has thrown more than 42% of his four-seamers either in the upper “shadow” or chase zones (zones 11, 12, 13, 21, 22 and 23 on this chart). Only 35% of his four-seamers came in those zones two years ago.

Sale’s four-seamer heat maps give a good sense of how his location has differed from two seasons ago.

Also encouraging: Sale’s four-seamer velocity is trending upward. Sale has thrown 29 four-seamers 95 mph or faster in his past two starts after reaching 95 mph only six times in his first two starts.

Sale fired 17 fastballs 95 mph or faster in his start against the Rays on Wednesday, marking his fifth-highest total in a start since the beginning of 2019.

His average velocity on four-seam fastballs has been above 94 mph in each of his past two starts, after the lefty averaged 93.2 mph and 92.1 mph, respectively, in his first two outings.

Sale hasn’t been shy about using his four-seamer, either. He’s thrown it more than half the time in 2021, easily a career high, and it has coincided with a drop in his slider usage to 26.6% (35% from 2017-19).

That tilt tells a story, not just about Sale's four-seamer but also his slider. Because while the overall results aren't an indicator, the fact is that his slider hasn’t resembled the force-of-nature pitch it was in the past.

Sale’s horizontal movement on sliders by year
2017: 13.1 inches (49% more break vs. average)
2018: 14.5 inches (46% more break vs. average)
2019: 14.0 inches (27% more break vs. average)
2021: 10.0 inches (19% less break vs. average)

Sale’s 32.5% whiff rate on sliders is a personal worst for the left-hander in any season under Statcast tracking (since 2015), as are his .236 expected batting average and .380 expected slugging percentage. From 2015-19, Sale had a 38.7% whiff rate and allowed a .146 xBA and a .224 xSLG on sliders.

Granted, it might not matter. If his four-seamer continues to profile as an elite pitch, Sale's slider at its current level could be good enough. It certainly hasn't slowed him down yet.

It will be something to watch down the stretch as Sale moves toward the ultimate test of his ace stature: a likely start in a win-or-go-home AL Wild Card Game.