Scotland's influential back-row forward Jason White is a major doubt for Saturday's Six Nations clash with Italy at Murrayfield.
The tough-tackling Sale Sharks star - who has been tipped by former Scotland captain David Sole to tour with the Lions this summer - has been ordered by doctor James Robson to undergo a scan on the knee he injured against Leeds Tykes last Friday.
The 26-year-old has been one of Scotland's best performers during their disappointing campaign so far but the prospect of him lining up alongside Simon Taylor was an exciting one.
Williams has already been forced to rule out Northampton back-row forward Ross Beattie, who is is remaining under the care of his club's medical team after he suffered a head injury in their loss to London Wasps on Sunday.
That has prompted Williams to draft in Edinburgh's Simon Cross and Glasgow's Paul Dearlove, who are both uncapped, to his squad for Tuesday's training session in Stirling.