Cristiano Ronaldo could miss Tuesday's Champions League game with Villarreal.
The Manchester United star limped out of Saturday night's 0-0 draw with Aston Villa in the closing stages with a leg injury.
Manager Sir Alex Ferguson said: "His leg is in a mess. He is doubtful for Tuesday."
United could have cut the deficit to six points after Chelsea and Liverpool were both held earlier in the day but a run of 14 successive victories over Villa came to and end.
Martin O'Neill's side defended well and deservedly ended their long-losing sequence against the Red Devils.
Ferguson continued: "I thought we were the better team apart from a 15-minute spell at half-time when Villa got the ball into our box from free-kicks and corners and caused a lot of problems.
"Apart from that, we weren't in danger. I was pleased with the effort we put in against an aggressive and determined team who defended for their lives.
"Our possession was good and we created the chances. Wayne Rooney and Ji-Sung Park had good chances.
"With the amount of possession we had at the end I thought another chance must come along but they held on."
