Everton striker Victor Anichebe was forced off during their victory over Bolton Wanderers yesterday.
Anichebe, who only returned to action in January after almost a year out with a knee injury, was hurt in a collision with Bolton defender Paul Robinson and replaced after just 14 minutes.
David Moyes said: "It looked as if he got a knee in the kidney or the ribs and they have sent him to hospital. We've not got the results yet."
The 21-year-old is now doubtful for the midweek trip to Manchester City.