The 49ers are unsure how serious running back Jordan Mason’s shoulder injury is after Thursday night’s win over the Seahawks.

Head coach Kyle Shanahan said after the game, after Mason went to the locker room with the initial injury, he told the staff that everything would be fine and he was cleared to return to the game. But after one play, Mason said he was in pain and was only kept available if the team needed him in an emergency, which it didn’t.

“He thought it would be okay, he went back in and it hurt so much, so he went back out, I think it was the first time. And then he was gone, just in case of emergency,” Shanahan said.

When asked if the injury is serious, Shanahan said that will likely be determined by additional testing on Friday.

“I don’t know. You never know. Adrenaline is a little different these days, during games. We’ll find out tomorrow, when that’s over, and then we’ll get x-rays.”

Mason is currently the NFL’s leading rusher, with 609 yards this season. He has more than filled in for the injured Christian McCaffrey, but it’s possible the 49ers will have third-string running back Isaac Guerendo for their big week seven game against the Chiefs next week.