PITTSBURGH -- The theme of this year for Mitch Keller has been change. He modified his arm action in the offseason to rediscover his velocity. He added a “sweeper” to his repertoire. He made his first Major League relief appearance. And in recent weeks, Keller’s pitch profile has been headlined by a new toy.

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Beginning last month, Keller has introduced and incorporated a sinker into his arsenal, which is slowly becoming his primary pitch. On Wednesday, Keller threw a career-high 42 sinkers as he pitched six innings of one-run ball with seven strikeouts in the Pirates’ 3-1 loss to the Tigers at PNC Park. Keller is still in the experimental phase, but his early returns with the sinker in his bag have been promising.

“I think I still don't have all the feel in the world for that pitch yet, but it's just such a good pitch with so much movement that it just gives me the confidence to attack the zone with it, even if I am … missing arm side or missing down with it,” Keller said.

With the caveat that correlation does not imply causation, Keller has thrown the ball well since introducing the sinker on May 18. In his past four outings, Keller has struck out 18 batters across 17 innings with a 2.65 ERA. Other factors have contributed to his success aside from the sinker -- Keller called his two appearances out of the bullpen against the Cubs and Rockies a “reset” -- but the sinker has quickly proven its effectiveness.

So far, opponents are batting just .150 (3-for-20) against Keller’s sinker. In his last start against the Dodgers, Keller generated an eye-popping 14 called strikes and whiffs on 34 sinkers. Keller didn’t get the same swing-and-miss on Wednesday, but of the nine balls the Tigers put in play, they ended up with only one hit.

What’s more encouraging is that Keller’s relationship with the sinker is still in its infancy. Keller has mentioned several times that he doesn’t have the best feel for the pitch yet, but he knows the pitch’s movement profile is good enough to compensate. As he becomes more acclimated with the sinker, the pitch could take another step.

The conversations about using the sinker picked up steam after Keller’s outing against the Reds on May 13, when he allowed five runs across 4 2/3 innings. Keller had dialogue not only with pitching coach Oscar Marin but with the folks at Tread Athletics, the training facility where he rediscovered his velocity this offseason. The conversations yielded a conclusion: Throw the sinker.

“The metrics were really promising and really exciting,” Marin said. “We just got to the point where the game was asking us to make an adjustment, and he made it. He had a pitch that, metrically, was unlike any other that he had with really good life. It’s one of those things that we decided as a group that he can move forward with.”

On the flip side, Keller’s usage of the four-seam fastball has plummeted. In his first seven outings of the season, Keller threw the fastball 57.3 percent of the time. Beginning on May 18, the day he first used his sinker, Keller has thrown the fastball just 21.3 percent of the time. Against Detroit, roughly 7 percent of Keller’s 95 pitches were four-seam fastballs, the lowest single-game usage rate of his career.

The numbers illustrate why Keller’s sinker usage is up and his fastball usage is down. Entering play, opponents had a .524 slugging percentage against the fastball, but only a .182 slugging percentage against the sinker. The fastball isn’t gone forever, but given how Keller’s last four outings have unfolded, the sinker appears to be king.

“I think it’s just all situational,” Keller said of his fastball and sinker usage. “If the sinker is playing or we feel like they’re on the sinker or we’re trying to throw a four-seamer in there to get a different look on the fastball there, then that’s what will happen. I’m going to let the game tell me what to do and not overanalyze how much I’m using one pitch. Just kind of let the hitter tell me what’s working and what I should throw.”