Larkin ran off the court clutching his shoulder after running into Sixers big man Joel Embiid in the first half. The Celtics dubbed Larkin's return questionable but coach Brad Stevens said after the game that Larkin was never available to return.
Stevens said Tuesday that Larkin was still undergoing tests and they would know more about his status at Wednesday's shootaround.
In his first year with the Celtics, Larkin, 25, has been a valuable backup ballhandler, especially since Kyrie Irving's season-ending knee injury. Larkin was critical early in the playoffs as the Celtics played without both Larkin and Marcus Smart, who returned from thumb surgery in Game 5 against the Bucks.
Larkin was averaging 4.1 points in 15.2 minutes a game for the postseason heading into Game 4.
"Next man up. Same as we've been doing all year," said Marcus Morris. "It has to be."