When Eduardo Rodriguez came up in 2015, he relied heavily on his four-seam fastball, throwing it 69% of the time, the greatest frequency in major-league baseball among pitchers with at least 100 innings. He mixed in a slider and and change and was mostly successful, putting up league-average numbers at just 22 years old.

Last year, Rodriguez missed time at the beginning of the season due to a dislocated kneecap. He never really got on track after that, getting off to a bad start that included a demotion to the minors. He improved somewhat by the end of the season but never really put up great numbers. Clay Buchholz beat him out for the third spot in the playoff rotation, and Cleveland’s sweep of Boston meant there was no need for a fourth starter.

It would have been fair, not so long ago, to regard Rodriguez as the fifth starter — behind David Price, Chris Sale, Rick Porcello, and Drew Pomeranz — on the 2017 Red Sox. Two months into the season, though — with Price injured, Porcello failing to repeat last year’s success, and Pomeranz continuing to struggle since last year’s trade — Rodriguez has been the team’s clear No. 2 option after the excellent Chris Sale. His 3.10 ERA is quite strong; his 3.34 FIP, 20% better than league average and bordering on ace level. It’s early, but Rodriguez has made a few different changes that have helped him become a more successful pitcher

We could simply write off last season as one plagued by injury and not bother to examine it any more thoroughly than that, but there’s some solid data in there. One thing to keep in mind — and something we should do for all pitchers, especially younger ones — is that it generally takes time to figure things out at the major-league level. Pitchers can be tinkering with different pitches, different pitch mixes, in order to get things right. A pitcher might not be pitching well at times, but he also might be taking the next step to becoming a better pitcher. Some of this should be taken with a grain of salt, but here is Rodriguez’s pitch mix over the last three seasons.

Eduardo Rodriguez Pitch Mix Season FA% FT% FC% SL% CH% 2015 69.2 % 1.4 % 11.6 % 17.9 % 2016 49.8 % 17.5 % 16.1 % 16.7 % 2017 58.0 % 10.0 % 3.4 % 8.7 % 19.9 % SOURCE: PITCHf/x

As noted above, Rodriguez threw his fastball a ton when he first arrived in the majors, mixing in his slider and change. Last season, he introduced a two-seam fastball and upped his slider usage. The slider was a pitch on which he worked in the minors and used it a lot when he came back up, but ultimately didn’t have a lot of success with it. Rodrgiuez’s best pitches in the majors have been the fastball and change. This season, he has gotten back to using those pitches the most. He throws both pitches for strikes, induces swings close to 50% of the time, and generates above-average whiff rates for each pitch (11% for the fastball and 22% for the change, per Brooks Baseball). Throwing those pitches more has yet to lessen their effectiveness. While increased use of his four-seam fastball is notable, as is continued use of the change, the decrease in the use of the slider is a big difference.

When discussing the slider, it’s important to note that the one Rodriguez uses now isn’t the same one he has used in the past. It’s slower by a few miles per hour, and the new version is one on which he was working throughout spring. With Rodriguez’s old slider, location often dictated that the pitch be used as an out pitch, requiring the batter to chase. The problem was that they mostly didn’t. Hitters swung at the pitch just 35% of the time last season per Brooks Baseball. Partly as a result, that version of the slider generated just 10% swinging strikes and was a ball more often than a strike. The heat map tells a pretty clear story (care of Baseball Savant):

Rodriguez threw a fast, sweeping slider out of the strike zone, but it didn’t really accomplish a lot, and he threw it about as often as his change. He’s using his slider a lot less this season, but it has been more effective. Here’s the same heat map for this season.

Hitters still aren’t offering on the pitch, swinging just one-third of the time and recording a pretty pedestrian 11% whiff rate, per Brooks Baseball. Because the pitch is in the zone, however, it’s getting a lot more strikes. Now, the pitch isn’t exactly a success, but it only needs to be less bad than it has been in past years to pair with the fastball and changeup. He’s throwing the pitch around 9% of the time so far this season, and that’s probably a good amount. It’s rare enough that the hitter isn’t going to be looking for it, and by throwing it for a strike, he should get some positive results out of it.

In the pitch-mix chart above, you’ll notice that Rodriguez has started experimenting with a cutter. That makes some sense. Armed now with a slower version of the slider, Rodriguez can use the cutter to enter some of the space formerly occupied by the slider. If the slider can’t be the occasional chase pitch, maybe the cutter can do that job. He’s only thrown the pitch 31 times this year, per Baseball Savant, but a bit under half have been balls, and he’s gotten swinging strikes on nine of those pitches. The heat map from Baseball Savant shows how it has replaced the type of pitch that the slider used to be.

Here’s that cutter:

Rodriguez took one bad pitch in his slider and turned into two pitches: a less-bad, slower slider that he can throw for strikes and a cutter that has more velocity than his old slider used to. His fastball and change are always going to be his go-to — 46 of his 55 strikeouts have come on those two pitches — but improving the rest of his repertoire a little bit and increasing or maintaining the usage of his best pitches could lead to more sustained success.

We don’t yet know how hitters might adjust to the slider and cutter, but if he’s throwing those pitches without much frequency, it could be hard to do. With David Price coming back and Chris Sale pitching great, even if Porcello doesn’t get back to the 2016 version of himself, Eduardo Rodriguez looks like a really good No. 3 starter for the Red Sox.