Sam Warburton is facing a minimum of four weeks on the sidelines due to an ankle injury, Cardiff Blues have confirmed.
Wales captain Warburton sustained the problem during the 13-6 defeat in the all-Welsh Pro12 clash with Ospreys on Saturday.
The flanker saw a specialist on Tuesday and will now begin his rehabilitation with a view to a return next month.
Cardiff's head of medical services John Miles said: "Sam has suffered a grade two strain to his right ankle and has now seen the specialist to start his recovery process.
"He will return to the specialist in a week's time for further assessment and currently we are working at a minimum lay-off of four weeks for a return to play."
Head coach Danny Wilson added: "We are hoping it is a four-week lay-off and Sam is a great professional who will work hard to get himself back as quick as possible. We wish him well with his recovery."
The news is a significant blow to the Blues, who are 10th in the Pro12 with one win from eight games and are third in Pool 3 in the European Rugby Challenge Cup with five points from their opening two matches.