Marlins reliever Michael Petersen on his UK roots, new pitch for 2026

It's difficult to hide when you're 6'7" and have a fastball that touches 100 mph, but even the most obsessed Miami Marlins fans may have missed seeing Michael Petersen on the team in 2024. The rookie right-hander made only five appearances for the Marlins that season, all of them in September after they had been eliminated from playoff contention. Claimed off waivers from the Los Angeles Dodgers, he posted a 4.76 ERA, 3.70 FIP, 4.76 K/9 and 4.76 BB/9. He was waived again at the beginning of the ensuing offseason.

In Petersen's second stint in Miami, he is quietly establishing himself as an important piece of the bullpen. Before we get to that, this is a good opportunity to dive into his unusual background.

Petersen was born in Middlesex, United Kingdom, located in a historic county in southeast England. He is the only British-born player on an active MLB roster, and the league's first since P.J. Conlon in 2018. Baseball is far down the list of popular sports in the UK, but Petersen's dad loved it and introduced it to the family. As a child, Petersen remembers playing with his twin brother, Thomas, and their friends in the house, creating balls out of wrapping paper and using a cardboard bat.

The Petersens moved to California and Michael played at St. Francis High School located in Mountain View, California. That's only about 40 miles south of Oracle Park, where the Marlins are currently playing against the San Francisco Giants. He was drafted four separate times, ultimately signing with the Milwaukee Brewers as a 17th-round pick in 2015.

Petersen remains proud of his British roots. He represented his homeland at the 2023 World Baseball Classic and was committed to do so again this year. However, the 31-year-old made the hard decision to sit out the tournament to prioritize making the Marlins Opening Day roster.

"I love Great Britain, and I love that team," Petersen told Fish On First. "I wanted to be a part of it, but I have this new pitch that needs to be worked on in front of my team, so I had to make that decision."

Team Great Britain went 1-3 and did not advance beyond pool play.

"I could've helped," Petersen said. "I was looking at it, and there were games that I wouldn't say they were a pitcher away, but I could have definitely helped. The guys there are still fantastic and I think they were dogs and it would have been nice about to go to battle with them, but making the team was my top priority, and I think being here really did help do that."

So far in 2026, Petersen has a 3.38 ERA, 3.43 FIP, 9.28 K/9, 3.38 BB/9 in nine appearances. His average fastball velocity of 96.9 mph leads all Marlins relievers. Opponents are hitting only .136 against his four-seamer with a 28.0% whiff rate. He ranks in the 85th percentile among MLB pitchers with a plus-three fastball run value, per Baseball Savant.

Petersen's "new pitch" is his changeup. After throwing only 11 changeups in the majors from 2024-25, he has already tripled that total this season. There's been only one hit off of it so far.

"Just something that goes the other way, so I can face both lefties and righties," Petersen said.

Petersen collected his first career save on April 8 against the Cincinnati Reds while Marlins closer Peter Fairbanks was on the paternity list. Petersen struck out two in a 7-4 win.

"It was awesome just to get that situation," Petersen said after that game. "A lot of trust from my team and my coaches to say 'hey, get out there and see what happens.'"

The all-time MLB saves record for a UK-born player is Tom Waddell with 15, so that is potentially within reach for Petersen a few down the road.

Petersen has thrown only one inning this week, so there's a high likelihood of him appearing during the remaining two games of the Giants series.