Scott McKenna faces at least three weeks out after suffering a hamstring tear in Aberdeen’s draw with Kilmarnock.
The centre-back came off after 27 minutes at Rugby Park and a scan earlier this week revealed the extent of the problem.
Derek McInnes’ defensive options have been restricted further by the news that Ash Taylor has also suffered a recurrence of his hamstring problem during his rehabilitation from a similar injury.
Taylor, who returned to the club in the summer, has not played since the first leg of the Europa League game against Chikhura Sachkhere last month, where he picked up the injury.
Dons boss McInnes had been hoping for some good news, in an area of the pitch where Aberdeen struggled with injuries last season.
He said: “We got the scan results back yesterday morning. As expected, there’s a bit of a tear but it’s not as bad as it could have been.
“It’s still going to be either three or four weeks but based on how he felt at the time, we were concerned it might be longer than that. We just need to deal with it and move on.
“Ash has had a little recurrence as part of his rehab, which is frustrating from our point of view. It’s not been great for us in one area of the team.
“It had a huge bearing on team selection last season and unfortunately we’re picking up injuries mainly to defenders this season as well.”
Zak Vyner, who joined the club on loan from Bristol City prior to the Europa League tie with Rijeka, came on for McKenna against Killie and can also deputise at right-back.
Mikey Devlin played 45 minutes for Aberdeen’s reserves in their 4-1 Reserve Cup win over Ross County on Tuesday, on his comeback from injury.
The ex-Hamilton Accies man has not played since the 2-1 win over RoPS Rovaniemi at the start of last month due to a hamstring problem but trained this week without any problems.
McInnes added: “We would have liked Mikey to play longer (on Tuesday) but with the news of Scott and Ash, we didn’t want to expose him to too much risk. But he’s trained this week and is doing fine.”
Midfielder Stephen Gleeson, who aggravated a knee problem during pre-season, has received some good news in his comeback in that he can start running again next week.