Back rower Jonathan Thomas is struggling to be fit for the Six Nations opener against England on 4 February
Thomas, 23, aggravated a hamstring injury in Ospreys' Heineken Cup loss to Stade Francais on Friday.
"He's has suffered the same hamstring injury that has troubled him before. It doesn't allow him to run over 50%," said Ospreys coach Lyn Jones.
"Is he a doubt for Twickenham? He's come off injured with two weeks to go. Make your own assumptions."