After previous Opening Day tease, Isaac Mattson has earned his spot with Pirates

BRADENTON, Fla. — Isaac Mattson will never forget lining up along the baseline at Fenway Park on April 2, 2021, the sights, the sounds and the Baltimore Orioles uniform that he wore.

But Mattson was also only part of the taxi squad, there in case of an emergency.

After pregame introductions, he had to change into normal clothes, not play a baseball game. Even when his teammates went to sign their names inside of the Green Monster, Mattson resisted — until the Pirates were in Boston this past August.

“I got a little taste of it,” Mattson said. “But there was definitely a piece of me that was like, ‘This isn’t for real yet.’ Still wanted to prove myself.”

Come Thursday at Citi Field, Mattson will get introduced and know it’s no longer a drill. No change of clothes necessary. He has certainly proven himself. In fact, between last year and how well he’s pitched this spring, Mattson has become a key part of the Pirates bullpen.

It’ll be the first-ever Opening Day for Mattson, who had a 2.45 ERA in 44 appearances last season for the Pirates, walking 19 and striking out 45 in 47 2/3 innings, blossoming into one of the key back-end bridges to Dennis Santana.

Amazingly, Mattson is one of eight first-timers on Pittsburgh’s roster, joining Braxton Ashcraft, Bubba Chandler, Hunter Barco, Spencer Horwitz, Jake Mangum, Billy Cook and Nick Yorke.

That amount of youth was a subject of conversation with general manager Ben Cherington before the Pirates' 5-2 loss to the Braves at LECOM Park, finalizing their Grapefruit League record at 17-13.

“The number’s higher than I anticipated given that a lot of them have had quite a bit of major league experience or were in the big leagues for a long time last year,” Cherington said."

At the same time, Thursday's game against the Mets brings plenty of excitement. It'll be different. For everyone.

"Every one of them earned it and deserve it," Cherington said. "They have a specific role on the team that they’re equipped for. It’ll be a nice moment for them. Opening Day in New York City. No joke.”

When it comes to Mattson specifically, there’s an interesting dichotomy here.

Mattson does his job with a metronomic level of consistency, and as a result the right-hander’s appearance tend to last about five minutes. This spring, he averaged under five pitches per out, which is remarkably efficient. Opposing hitters batted just .083 against Mattson, who didn’t allow an earned run during Grapefruit League play.

(For context, Paul Skenes had an .095 batting average against in Spring Training.)

“It’s pretty cool to be able to put in the work in the offseason and get an opportunity to break with the team,” Mattson said.

That work has primarily involved diversifying his pitch mix. After throwing his four-seamer 78.5% of the time last year, Mattson has thrown his changeup more this spring while also working in more curveballs and sliders.

We’ll see if it sticks during the regular season. The four-seamer — due to how well Mattson hides the ball — was nearly unhittable in 2025, netting a .191 batting average and .287 slugging percentage against.

“Just working on offspeed stuff, trying to figure out a weapon versus lefties,” Mattson said. “Then being able to throw it in-zone with confidence and execute it to good spots with two strikes. I’m sure the fastball will still be a focal point. But I wanted to spread the deck a little bit and keep guys on their toes.”

In seven spring outings, Mattson went 2-0, walked one and struck out eight in 7 1/3 innings. The 22 outs he recorded required just 107 pitches.

Mattson has become sort of a right-handed, modern-day Tony Watson, where a bathroom break might cause you to miss his entire outing.

That’s obviously a good thing.

“Practice should be tougher than the games,” Mattson said. “I’m a firm believer in that. I know every outing isn’t going to be as quick or easy as some of the outings have been here. But from a confidence standpoint, it's nice to know I can get the job done quickly and pass it on to the next guy so we can win the baseball game."

Jason Mackey: Jason.Mackey@pirates.com and @JMackey_PGH.