Cueto returned to Cincinnati on Saturday to be examined after leaving the series opener at Texas on Friday night because of tightness in his right lat muscle.
Manager Dusty Baker hadn't yet gotten a report about that exam before Saturday's game, but said the Reds would be making a move. He called the setback for Cueto disheartening.
Right-hander Logan Ondrusek had joined the Reds in Texas from Triple-A Louisville.
Cueto, the Reds' opening-day starter, was on the DL from April 15 to May 19 with a strained right lat. He missed the first half of June with what was called a strained right shoulder, though it was related to the problem in his upper back.