Trying to diagnose what's wrong with Yankees reliever Mark Leiter Jr.

As Texas Rangers' No. 9 hitter Carson Kelly sent Mark Leiter Jr.'s inside sinker high over the Globe Life Park wall for a home run, it was clear something was up with the New York Yankees' recent trade acquisition.

Kelly's blast was the second home run Leiter had allowed in that Aug. 11 appearance in Texas, with Nathaniel Lowe to going yard just three batters earlier. This came afer Leiter surrendered just two home runs total his the previous 42 1/3 innings thrown in 2024.

Leiter Jr., acquired at the trade deadline for minor leaguers Jack Neely and Ben Cowles, had gotten off to a decent start in the pinstripes, pitching to a 1.50 ERA while striking out eight batters in six innings through his first six appearances.

But since from that fateful Aug. 11 blowup onwards, Leiter has been borderline unplayable. His 9.00 ERA has been the worst out of all Yankees with more than two innings under their belt during that span, and he has allowed four additional home runs since that evening in Texas.

A quick peek at Leiter's Fangraphs page shows that two out of his three true outcomes have not changed since his successful first half in Chicago. His strikeout rate is down from his elite 34.9% mark, but at 27.9% he's still well above average in that department, and certainly one of the best swing-and-miss relievers the Yankees have. His walk rate has actually gone down in New York, from a decent 8.6% to an even better 7%.

It doesn't take a genius to realize that it's his third true outcome, the home run ball, that has been Leiter's undoing in the Bronx. But with his unchanging ability to generate swings and miss bats out of the zone (and his previous propensity for doing the same inside it), there must be something that has changed about his pitching style that has brought about this 180-degree shift in results on contact.

On Aug. 11, Leiter's most-used pitch was his sinker. Sitting around 92 MPH on average, and dropping 3.4 inches less than the average sinker, it's not his bread and butter pitch by any means. But until that day, he hadn't allowed a homer on the pitch all season. He relied mostly on his splitter early on in his outing that day, but after allowing Lowe to homer on a decent splitter down and in, he seemed to lose faith in the pitch, shifting over to his sinker, then leaving one over the heart of the plate that Kelly drove into the seats.

Three out of Leiter's four home runs since have been given up on that sinker, which illustrates a disturbing trend: hitters seem to have recognized that his fastball is nothing special and have started to sit on the pitch more instead of whiffing at his very good secondary pitches.

In August, batters swung at just 43% of Leiter's pitches, a season low. As of Sept. 14, batters have not whiffed on a single Leiter sinker in the month of September, as he's gone from painting the black with them to either missing just outside or leaving them right over the heart of the plate.

Even more stark is batters' reactions to his splitter, as they've gone from whiffing at it 64% of the time in July, to 54% in August, and all the way down to 33.3% in September. It has gone from an elite swing-and-miss weapon that Leiter could rely on in any count, to a pitch batters just spit on in hopes he'll abandon it for the sinker.

The good news is that Leiter's overall contact rate has not changed drastically since his trade to the Yankees. It's just the quality of contact that has. The only real change in pitch mix that Leiter has undergone is dropping his sinker usage in favor of his splitter in September, which shows that the problem has been recognized, but doesn't necessarily provide a long-term fix.

In the long run, this problem seems to be a mechanical issue that is affecting Leiter's sinker location. Although he is clearly a splitter-first pitcher, he still needs some type of fastball in order to succeed, and until he goes back to being able to dot his sinker on the black, Leiter will likely stay mired in his struggles.

The Yankees have control over Leiter until the end of the 2026 season, so there's plenty of time for both sides to work together to solve the issue and get some value out of the trade. But as far as the 2024 stretch run and postseason are concerned, there is a lot of work that has to be done before Leiter is the bullpen asset he was acquired to be.