Rob Thomson’s loss is Don Mattingly’s gain.

Thomson was fired as manager of the Phillies on Tuesday with the club off to a 9-19 start and in last place in the National League East.

Thomson, who led Philadelphia to the pennant in 2022 after taking over midseason for Joe Girardi, is now himself being replaced during this campaign by a baseball lifer in Mattingly, who has plenty of experience as a skipper.

Mattingly, a six-time All-Star with the Yankees as a player, previously led the Dodgers (2011-15) and Marlins (2016-22).

He spent the last three seasons as the Blue Jays’ bench coach and joined the Phillies — where his son, Preston, is the general manager — in the same role before this season.

It is believed that the Mattinglys are now the first father-son manager-GM duo in MLB history.

After ex-Red Sox skipper Alex Cora turned down Phillies president of baseball operations Dave Dombrowski’s overture to take over, Mattingly accepted the role.

“I think we’ve had multiple issues,” Mattingly told reporters before Tuesday’s game against the Giants. “It’s not just hitting — we need to play better baseball. And it’s pretty much as simple as that, we’re 9-19 because we haven’t played good enough baseball. And that’s a simple thing for me.”

Mattingly brings in a 446-363 regular season record, including three straight 90-win seasons and NL West titles from 2013-15 with Los Angeles. He won NL Manager of the Year in the COVID-shortened 2020 after leading the Marlins to the playoffs for the first time since 2003.

After leaving the Blue Jays following their World Series heartbreak against the Dodgers, Mattingly told The Post’s Jon Heyman that he didn’t have intentions of retiring but did not feel that the one managerial opening at the time — the rebuilding Rockies — was a good fit for him. He also revealed that he was trying to find the right “balancing act” for himself and his family.

“If I talk about it right now, I’m like, ‘the last thing I wanna do is be on the road,’” he said. “But you never know how you feel in a month from now or whatever. So kinda those are the things I wrestle with. This is really that balancing act of family and still being a part of the game, which I still like what I’m doing.”

His arduous Phillies task begins Tuesday night as he hopes to lift a team that ranks 29th in MLB in OPS (.656) and 28th in ERA (5.13).