Rhys Williams could be in danger of missing the 2014 World Cup with the Australian national team.
The Middlesbrough captain, who is tipped to be in the Socceroos squad in Brazil, sustained a freak injury during his side's 0-0 Championship draw with Wigan Athletic on Tuesday night.
Williams was stretchered off in pain at the end of the first half at the Riverside Stadium with Boro boss Aitor Karanka expecting the worst for the 25-year old.
"They will check him and we will know more soon but it looks like a very serious injury for Rhys," said the Spanish head coach.
"His face and the doctor's face were not good. He was crying and I feel sympathy for him.
"It is very bad injury for him. I hope he will recover as soon as possible because he had a big chance to play in the World Cup so that will be a big incentive for him.
"All our feelings are with him, mine, the team's, the staff's. He will get all the support we can give him."
Football Federation Australia also confirmed the injury suffered by Williams, who also missed the 2010 World Cup because of a pelvic issue.
"Football Federation Australia medical staff has been informed by Middlesbrough that Socceroo defender Rhys Williams has suffered an Achilles injury and are awaiting more information regarding the extent of the injury," an FFA spokesperson said.
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