The 25-year-old Australia international suffered a suspected ruptured Achilles tendon during Tuesday night's 0-0 home draw with Wigan and is due to visit a specialist in London this week.
As a result, the luckless Williams, who missed the 2010 World Cup finals in South Africa through injury, is likely to have to sit out the rest of the season and that would end his hopes of making his country's trip to Brazil this summer.
Central defender Jonathan Woodgate was unavailable for the Wigan game with a thigh problem and is likely to be absent once again while midfielder Faris Haroun has had his contract cancelled by mutual consent.
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