The 23-year-old, who missed 11 months of football last season due to a pelvic problem, was ruled out of Tuesday's game to a related groin injury.

Williams has previously revealed he may never fully recover from the pelvic injury and must constantly monitor his workload.

Mowbray said after Tuesday's defeat that the Australian defender/midfielder needed a break after playing in both of the Socceroos matches against Malaysia and Oman in quick succession last week.

"Rhys's injury has been coming for few weeks and playing two games in quick succession at full-back for Australia made it difficult," Mowbray said.

"He was unavailable to play, he was too sore and we have to rest him and get him back soon.

"We've been trying to manage him well and he's used his quality and brain and generally doesn't have to work too hard. But he's been away with his national team and running up and down the wing.”

Mowbray added that the side missed Williams as they slumped to their first league defeat of the season.

The Boro boss also dropped Socceroos striker Scott McDonald to the bench in the game where he remained an unused sub, having failed to score since mid-August.

"Sometimes you need to break the pattern with players and that's what I was trying to do with Scott, give him a mental breather," Mowbray said.

"He's worked his socks off for the team and he's one of the reasons we've done well so far this season. Ultimately he needs to score some goals, so we took him out of the firing line and he'll be back soon."

Elsewhere, in the Uefa Champions League overnight, Scott Chipperfield was an 85th-minute substitute as FC Basel slipped to a 2-0 defeat at home to Benfica.