Ulster's Tommy Bowe and Louis Ludik have been ruled out for the remainder of this season.
Bowe returned after six months out against Zebre in April but missed the wins over Leinster and Ospreys because of a recurrence of a knee injury.
An exploratory operation has revealed no new damage but the Ireland winger will not feature again this season.
Ludik will also miss the Pro12 semi-final against Leinster on 20 May as he will undergo groin surgery on Thursday.
Ulster are already without Cardiff Blues-bound Nick Williams for the rest of the campaign, but Les Kiss' side did not pick up any further injuries in Saturday's win over the Ospreys.
Bowe sustained his serious knee injury in Ireland's World Cup quarter-final defeat by Argentina in October.
The Ulster winger also looks likely to be ruled out of Ireland's Three-Test tour of South Africa next month.