The Manchester United winger collided with an opponent during a practice match overnight and left the field with an ice pack wrapped around his left foot.

He was seen walking without too much difficulty after training and coach Luiz Felipe Scolari will hope he recovers in time for the crucial match in Belgrade.

Portugal team spokesman Onofre Costa said: "At this time we don't know if he [Ronaldo] will play.

"He sprained his heel - it doesn't look very serious but we are not sure if he will be fit."

Portugal are currently third in Group A, level on 10 points with fourth-placed Serbia. Poland lead the way on 13 points, ahead of Finland on 11.