Williams, 25, went down just before the break in a 0-0 draw at home to Wigan Athletic, causing a lengthy stoppage. A leg brace was applied and he was provided with oxygen on the pitch, prompting fears of a serious tear.
While Middlesbrough are yet to comment on the severity of the injury, it appears the versatile player – capable of operating at centre-back, full-back and in defensive midfield – is now in serious doubt for Australia's World Cup campaign, which begins against Chile on June 13.
It would be a cruel blow for Williams, who also missed out on a place in the 2010 World Cup squad due to a pelvic problem.
The former Wales under-21 international had been linked with moves to English Premier League club Swansea City and Scottish Premiership champions Celtic in recent weeks, but any chance of a switch to a higher level is likely to have now been scuppered.
Adding to Postecoglou's defensive woes is the fact that captain Lucas Neill is still without a club, while his fellow centre-back Sasa Ognenovski has been out of favour at Umm Salal and is set to leave the Qatari side, with his destination yet to be determined.
The Socceroos have already lost key forward Robbie Kruse for the World Cup after he suffered an ACL tear during training with club side Bayer Leverkusen.
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