After a remarkable reinvention, Red Sox reliever John Schreiber has batters shaking their heads

John Schreiber has struck out 25 and walked just 5 in 21⅓ innings.Jim Davis/Globe Staff

Red Sox reliever John Schreiber had a surprise for his former Tigers teammates Monday. The 28-year-old righthander unleashed a 97-mile-per-hour fastball — the hardest pitch he’s thrown in his career — while recording a pivotal out in his team’s eventual 5-2 victory.

“A couple of guys said their jaws dropped,” Schreiber said. “It’s pretty funny.”

It’s hard to blame the Detroit players. As a Tigers pitcher, Schreiber averaged 90.8 on his four-seam fastball while recording a 6.28 ERA over 28 big league appearances in 2019-20.

He relied chiefly on the deception of an unconventional delivery with a low release point rather than stuff. Over those two years, he threw one pitch over 94 m.p.h. — a mark that now represents his baseline velocity. In February 2021, no one blinked when Detroit designated him for assignment and the Red Sox claimed him off waivers.

Nor did it make any waves when the Red Sox designated Schreiber for assignment at the end of spring training in 2021. No one claimed Schreiber, who passed through waivers and was outrighted off the 40-man roster to Triple A Worcester.

But with the WooSox, and with help from members of the coaching and support staff, Schreiber began a transformation that has now positioned him as a critical late-innings contributor.

“It’s been an amazing process,” said Red Sox minor league pitching coordinator Shawn Haviland.

John Schreiber has emerged as an important part of the Red Sox bullpen.Jim Davis/Globe Staff

When Schreiber landed with the Sox, he almost exclusively used a four-seamer and slider, with his low arm slot creating enough deception on the slider to make it a potentially useful building block.

“He throws unconventionally,” said Haviland. “He kind of bends over and then sort of pops over the top a little bit. It definitely is a weird at-bat. Just playing catch with him, you can kind of see the unquantifiable deception that he brings.”

But the Sox wanted to pair deception with stuff. Schreiber threw the occasional sinker and changeup with the Tigers but without any great commitment to either pitch. And, of course, he didn’t throw hard at all.

So, what happened?

“He’s the model for buying in and just working harder,” said WooSox pitching coach Paul Abbott. “He was just like a lot of guys who would throw six pitches off of a mound and then just walk away. It was like, ‘Is that all we’re doing?’ ”

Among pitchers with at least 20 innings this year, Schreiber ranks in the top 10 in ERA (0.84) and OPS allowed (.431). He has struck out 25 and walked just 5 batters in his 21⅓ innings.Barry Chin/Globe Staff

At the time, Schreiber told Abbott he wanted to keep his arm fresh for games. Abbott suggested that Schreiber focus more on between-outings work to have better stuff when competing.

The two worked to sharpen his slider (Haviland estimated that it has gained 3 inches of break) and to develop both his sinker and changeup — pitches that would allow him to attack both the top and bottom of the strike zone.

Schreiber also took the change of organizations as an opportunity to alter his off-field approach, working with the Sox’ strength and conditioning staff.

Two months after Schreiber arrived, said Abbott, “I had to take the ball away from him because he’s working so hard in the bullpen.”

Schreiber started dominating down the stretch in Worcester last season. He had a 0.64 ERA with no homers allowed in his last 28⅓ innings. Though he appeared in just one big league game — allowing one run in three innings while striking out five as an emergency call-up amid last September’s COVID outbreak — Schreiber understood how far he’d come and the possibilities presented by his progress.

Red Sox coaches and officials encouraged him to be ready to hit the ground running in spring training in 2022, and Schreiber took the charge to heart.

He opened eyes in spring training, and was one of the last cuts. He was not discouraged by an assignment to Worcester to open the year, and dominated in Triple A. He was ready when the Sox summoned him, first for a series in Toronto in April, then again in May.

Schreiber sprints in from the bullpen to take the mound against his old mates from Detroit earlier this month.Jim Davis/Globe Staff

While Schreiber primarily employs his four-seamer and slider — the mix that initially got him to the big leagues with the Tigers — increased velocity coupled with liberal usage of his sinker (a pitch he’s thrown 23.4 percent of the time) has left batters shaking their heads following check swings, chases, and broken bats.

“His stuff is dirty,” said catcher Christian Vázquez.

Among pitchers with at least 20 innings this year, Schreiber ranks in the top 10 in ERA (0.84) and OPS allowed (.431). He has struck out 25 and walked just 5 batters in his 21⅓ innings.

In the process, Schreiber has emerged as one of Alex Cora’s most trusted late-innings options. This week, he worked in three straight games while recording two eighth-inning holds (one against the Tigers, one against the Cardinals) before recording the final out against the Tigers by retiring Miguel Cabrera for a save Tuesday.

The fact that the save came not just against his former team but also the team for whom the Michigan native rooted as a kid afforded a reflection on his astonishing progress.

“I’ve come a long way over the past couple of years just from learning from these guys,” said Schreiber. “I’m extremely grateful to get to where I am today.”

The Sox, meanwhile, harbor considerable appreciation for the pitcher Schreiber has become and how he arrived at this point.

“I really tip my hat,” said Abbott. “I’m extremely proud of him, because he worked his butt off.”