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.
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.
