The Perth-born 24-year-old defender/midfielder, who has had a bad history with injury, had to be stretchered off after 74 minutes in Tuesday's 2-0 victory over Gillingham in the Capital One Cup.

Williams underwent scans on Wednesday with Boro boss Tony Mowbray revealing the extent of the damage to the Evening Gazette on Thursday.

"Rhys had his scan yesterday morning and has damaged ankle ligaments, which is what was suspected," Mowbray said.

“He will see a specialist to decide whether he needs an operation on that or not. You would have to say he will be set for a considerable time out.

“It is going to be a few months minimum and I would imagine it would be a bit more. It’s not only a blow to Rhys but to the club and the team.”

Williams missed most of the 2010-11 season and the 2010 World Cup after suffering a long-term pelvic injury which he later admitted threatened his career.

Earlier in the week, Boro boss Tony Mowbray had admitted he had pondered resting Williams for the midweek game.

Williams' injury is a major blow for promotion hopefuls Boro who sold defender Joe Bennett to Aston Villa this week too.