Stanton suffered the injury while trying to stretch out a double with two outs in the ninth inning against the White Sox on Saturday. He was ultimately thrown out on the play, and Miami dropped the game 8–7.
Mattingly called the MRI results “serious.”
Marlins president of baseball operations Michael Hill revealed to reporters on Sunday that Stanton sustained a Grade 3 strain to his groin. He believes the best-case scenario is a six-week recovery time.
That timeline would put him on track to return against the Braves on Sept. 25 and would leave the team with seven games left in the regular season. If he is unable to return, it would be the third straight season that Stanton has finished on the DL.
The 26-year-old has also dealt with hip issues this season, although it is unclear if the injuries are related.
Stanton is batting .244/.329/.496 with 25 home runs and 70 RBIs this season.
The Marlins (61–56) are tied with the St. Louis Cardinals for the second wild card spot in the National League.