Jonathan Thomas drops out of the 23 for Saturday night's Magners League Welsh derby against Cardiff Blues at the Liberty Stadium.
The Welsh international back rower had withdrawn from the squad with a tweaked hamstring, the medical team at the Ospreys opting not to risk him. He is expected to be available for the Ospreys next fixture, at Newport Gwent Dragons in two weeks.
His place on the bench has gone to Tom Smith.
Cardiff Blues have also made a late change, Chris Czekaj replaced by Dan Fish at fullback.