Sir Alex Ferguson estimates Rio Ferdinand will be out of action for two weeks after missing Manchester United's victory over Birmingham on Sunday afternoon.
Ferdinand was forced to pull out with a thigh injury, with Ferguson claiming before the game that his £29.1million defender will be going for a scan on Monday morning.
However, the United boss has now put a timescale on the problem, which would make the 30-year-old a doubt for England's World Cup qualifier against Croatia on September 9.
"I would imagine Rio will be out for two weeks," said Ferguson.
With Nemanja Vidic already sidelined and Jonny Evans limping out of Sunday afternoon's 1-0 win over Birmingham, Ferguson is short of defensive cover ahead of Wednesday's trip to another newly-promoted side, Burnley.