Ashley Beck, the Ospreys centre, has been ruled out for up to four months.
The Wales international has undergone surgery after picking up a shoulder injury in pre-season training.
The 27-year-old could be out until the start of 2018 and will miss the first half of Ospreys Pro14 season as well as their opening European Champions Cup matches.
Beck played 19 times for Ospreys last season after missing nearly all of the 2015-16 season through injury.
Ospreys' Wales forwards James King and Dan Lydiate are making good progress following injury and should return in September.
Lydiate has not played since picking up knee and shoulder injuries in Wales' 27-13 autumn series win against South Africa on 26 November last year.
King missed Wales' summer tour matches against Tonga and Samoa after injuring and ankle in the Pro12 semi-final defeat to Munster in May.