Fellaini picked up a knock during the 2-2 draw with Wigan Athletic last weekend and although he managed to complete the game, the significance of his injury only became apparent 48 hours later.
He was withdrawn from the World Cup qualifiers against Serbia and Scotland and had the injury scanned in Belgium.
The Blues medical team have reviewed his scans and will assess him fully when he returns to Finch Farm on Monday.
Seemingly we won't know more until next week, but it is interesting that the injury happened against Wigan. Fellaini is prone to go into the red (negative fantasy points) when the game isn't going his way, but now we can add the injury to the list of reasons why.