"He took a step back to refine his lower-body mechanics," Detroit manager Ron Gardenhire said after Thursday's 4-4 tie with Boston. "We have no timetable when he's going to get back on the mound."
Gardenhire declined to provide additional details about the decision.
Fulmer won AL Rookie of the Year honors in 2016 but has struggled to return to that form. He was 3-12 with a 4.69 ERA last year, his season cut short in mid-September because of knee problems.
Outfielder Nicholas Castellanos, who had a bruised left hand, is scheduled to take batting practice Friday. Gardenhire said if soreness persists, Castellanos might be examined by doctors.