The Tigers transferred Horn (elbow) from the 15-day injured list to the 60-day IL on Wednesday.
Horn is coming back from offseason arthroscopic left elbow surgery and was pulled off his rehab assignment last week after incurring a setback. Per MLB.com, Horn recently received a cortisone injection in his elbow, so he'll presumably be shut down from throwing for at least a few days while he waits for the shot to take full effect. The reliever could eventually resume his rehab assignment within the next few weeks, but he won't be eligible for activation until late May.