The team ruled Larkin out for Game 5 of its second-round playoff series against the Philadelphia 76ers on Wednesday night.
"I don't anticipate [Larkin] back anytime soon," Stevens said.
Stevens said surgery likely won't be needed, and Larkin will work with Boston's training staff to try to strengthen the shoulder, which he injured in Game 4 against the 76ers on Monday.
Boston is already playing without All-Star point guard Kyrie Irving, who had season-ending knee injury last month.
"We're just going to have other guys step up. We're used to that," Stevens said at Wednesday morning's shootaround. "At the end of the day, that is what it is. [Larkin has] played really well in this series. And he's done a lot of great things for us all year and through the playoffs."
Larkin played just two minutes Monday before suffering the injury when he collided with Philadelphia's Joel Embiid.
"We've just got to do what we can," Celtics starting point guard Terry Rozier said. "This has been our team. We had to get Game 1 without Jaylen Brown, so the story continues. It's just the way it goes."
Larkin was critical early in the playoffs as the Celtics played without Irving and Marcus Smart, who returned from thumb surgery in Game 5 of Boston's first-round series against the Bucks. Larkin was averaging 4.1 points in 15.2 minutes per game this postseason heading into Game 4.