The Perth-born 23-year-old, who has nine Socceroos caps, injured his ankle in the 2-0 loss against Leeds United earlier this month.

Boro initially said they'd monitor the problem and hoped he'd be fit to face either Birmingham and West Ham earlier in the week but he missed both games.

Mowbray has now revealed Williams has suffered ligament damage which could see on the sidelines for the best part of the rest of the season.

"He's damaged ligaments in his ankle, so it's never a week or so," Mowbray told BBC Tees. "Let's hope we can see him again this season and he's got a part to play."

Mowbray confirmed Williams was at a minimum several weeks away for the promotion hopefuls.

"He'll have two or three weeks in a cast, and then we'll reassess once he has that off and starts running to see how bad it is," he said.

Fellow Australian Scott McDonald is also struggling to return for Boro before the end of the season due to a knee injury.