Ben Foster was ruled out of England's World Cup double-header against Ukraine and Belarus with a chest injury.
The Football Association did not specify Foster's injury when the squad was released last night, but now United have confirmed the 26-year-old is being "investigated" for an injury sustained during Saturday's 2-2 draw with Sunderland, although not serious enough to prevent him completing the match.
Conspiracy theorists had suggested an injury was merely the excuse for omitting Foster when the decision had been taken on the grounds of form.
Foster has been blamed for Sunderland's second goal at Old Trafford, when Kenwyne Jones showed more strength in an aerial duel.
This came two weeks after what Sir Alex Ferguson described as a "difficult" afternoon against Manchester City, when Foster was blamed for two goals, losing the ball to Carlos Tevez for Gareth Barry's opener, then going down far too quickly for Craig Bellamy's second equaliser.
In between, Foster did keep a clean sheet at Stoke, although he was also left out of matches against Wolves and Wolfsburg as Tomasz Kuszczak was given an opportunity.
Foster's chances at United look like being severely limited after the international break as Dutch veteran Edwin van der Sar is due to return after breaking two fingers in a pre-season clash with Bayern Munich in July.