Cashner was three-quarters of the way through a bullpen side session on Monday in Mesa, Ariz., when he felt discomfort behind his right shoulder.
Cashner's rehab will be similar to what he's gone through the past six weeks. There's no new timetable for Cashner to start throwing again.
"It's a tough day for the Cubs and Andrew," general manager Jim Hendry said. "He had sailed through his rehab up to this point and was just a few pitches away from being sent out on a rehab start."
Cashner was headed home to Texas on Tuesday for personal reasons unrelated to his injury, but was evaluated by the team's medical staff before he left.
Both Cashner and fellow Cubs starter Randy Wells went on the DL on April 8. Wells suffered a right forearm strain. Wells' first rehab start will be Tuesday in Peoria, Ill.
Bruce Levine covers the Cubs for ESPNChicago.com.