The Black Cats' leading scorer was injured just minutes into Scotland's World Cup qualifying defeat to Wales on Friday and was stretchered off.
Sunderland still do not know the extent of the injury as swelling around the injured area has to go down before an accurate scan can take place, but fears are growing that it could be a bad one.
Sunderland insist it is too early to speculate on how long the striker will be out but that has not stopped the rumours, with some reports claiming he faces up to six weeks on the sidelines.
That would be the last thing Black Cats boss Martin O'Neill needs to hear with the club slipping towards the relegation places.
He will be hoping Fletcher recovers sooner rather than later, but he is certain to miss the home clash with Manchester United on Saturday lunchtime.