On May 13, my friend asked me if I wanted to go to a game at Fenway Park the following day. I said sure – I hadn’t been to a game in awhile and I was only on the hook for the face value of the ticket. 

By the sixth inning, it wasn’t much of a game, as the Sox were beating up on Luke Hochevar. But that thing-that-you’re-not-allowed-to-name-while-it’s-happening was happening. By the seventh inning, most of the crowd realized that Jon Lester hadn’t yet surrendered a yet. Nine outs later, Lester put the exclamation point on his victory over cancer by completing the fifth no hitter by a left-hander in Fenway Park history.

But Lester’s no-hitter may only be the tip of the iceburg for him. In fact, it appears that Lester has been steadily improving throughout the entire season, and may now be one of the best pitchers in the American League.

When I went to that fateful game on May 14, Jon Lester’s season statistics were respectable. He had a 3.95 ERA through 57 innings, although he also sported a miserable 33/29 K/BB ratio. He was saved by the Sox’s defense – his BABIP was .280 – and his ground ball tendencies. At the time, Lester had induced grounders on 53% of his balls in play. He had thrown 61% of his pitches for strikes, and 10.2% of those strikes were swings-and-misses.

His fastball, cutter, and curveball velocities were all 2-3 MPH faster than they had been the year before, but Lester had seemingly struggled with both his control (the ability to throw strikes) and command (the ability to throw quality strikes), as evidenced by his poor K/BB ratio.

It appears that Lester’s control and command have been steadily improving throughout the season. While his no hitter may be one of the pinnacle achievements of his career, it’s quite possible that Lester is a better pitcher today than he was on May 14. His control is certainly improved: in his first ten starts, Lester threw 61% of his pitches for strikes; in his following 12 starts, he has thrown 66.4% of his pitches for strikes. Predictably, he walk rate has fallen dramatically: he walked 4.58 batters per nine innings in his first ten starts; in his subsequent 12 starts, he has walked only 1.71 per nine. That’s a major improvement.

But if we look even deeper we can see that Lester’s control did indeed improve, and his command is coming along too. In fact, in Lester’s last four starts, he appears to once again have taken an additional step forward. Let’s break Lester’s season down into three segments:

As you can see, Lester’s ERA has steadily improved. His walk rate improved starting with his no hitter, and his strikeout rate has constantly climbed throughout the year. Meanwhile, he has been able to hold his groundball rate fairly constant (and note how low his fly ball rate has been in his last five starts).

It’s also important to note that two of Lester’s last four starts came against the Yankees, so it’s not like Lester is benefiting from a particularly easy stretch of the schedule.

Interestingly, Lester has been very unlucky on balls in play recently:

Despite his 2.15 ERA in his last four starts, Lester has been giving up an inordinately high number of hits on balls in play. However, Lester has also given up an inordinately low number of home runs:

Lester is a groundball pitcher, meaning he shouldn’t give up many homers overall. That being said, his rate of homers per fly ball is inordinately low this year – overall, only 8.5% of Lester’s fly balls have become homers. This is likely to regress to the mean.

Finally, let’s take a closer look at Lester’s command. It appears that Lester improved his control rather quickly, but his command has improved gradually this season. He dropped his walk rate, starting with his no-hitter, and has maintained a low walk rate throughout the rest of the season. But his strikeout rate has been steadily rising, as I’ve shown. And, the quality of the strikes that Lester is throwing appears to be improving as well.

Note that Lester isn’t just throwing more strikes (improved control), he’s getting more swinging strikes (likely due to improved command). The amount of swinging strikes, in particular, has jumped in his last four starts. Lester’s walk rate was virtually the same in "next 7 starts" as it was in "last 5 starts," but his strikeout rate improved a lot in "last 4 starts." This suggests that his command has improved now, as well as his control.

Pitch f/x provides an excellent visual for what I’m talking about. Here is a plot of all of Jon Lester’s pitches from a start on April 19 against Texas: 

I added the smaller box in an effort to distinguish "quality" strikes from strikes that catch the heart of the plate (this may not apply to curveballs, but Lester throws mostly fastballs and cutters anyway). Now compare this to a plot from Lester’s most recent start, July 27 against the Yankees:

Note how many pitches are in the strike zone but not in the smaller box. Also notice the fairly large space in the middle of the smaller box that is completely devoid of any pitches whatsoever. It’s somewhat difficult to tell the difference because Lester threw more pitches overall in the second start than the first, but you can see pretty clearly that not only did Lester throw more strikes in the second graph I’ve shown, but he threw more quality strikes as well. 

Jon Lester’s velocity and sink have returned to his fastball and cutter this season. As the season has progressed, we can see clear evidence of improved control (he has lowered his walk rate) and improved command (he has raised his strikeout rate and has gotten more swings-and-misses). Lester is seemingly now above-average in the three most important aspects of being a pitcher: keeping the ball on the ground (and thus, in the park), preventing walks, and accumulating strikeouts. If these improvements are real – and we have no reason to think they’re not – Lester is set to become one of the best pitchers in the American League.

No wonder the Red Sox wouldn’t deal him this offseason.