Johnson sprained the AC joint in his right shoulder in Boston's stunning come-from-behind victory in Game 3. Stevens said he was scheduled for an MRI exam.
The 6-foot-9, 250-pound Johnson drew the start in Sunday's victory, which saw the Celtics storm back from a 21-point deficit in the second half to win on Avery Bradley's 3-pointer at the buzzer.
Johnson played just over nine minutes in the contest before he was hurt. He averaged 6.5 points and 4.6 rebounds during the regular season for Boston but is struggling in the playoffs with 2.7 points and 2.1 boards per game.
The Celtics, who trailed the best-of-seven series 2-1, already are without leading scorer Isaiah Thomas, who was ruled out of the playoffs after aggravating a hip injury during Game 2 of the conference final.