The England midfielder sustained an Achilles injury in training on Friday and missed the trip to Leeds on Saturday.
Physio Derek Wright confirmed: "Yes, it's a bit worrying but we will not know the real extent of JJ's injury until we get the results of the scan tomorrow.
"The best scenario is that it is only strained and that he will be out for a week to 10 days.
"But if there is a tear in the injury then we will be talking about weeks."
If JJ is ruled out for any period of time it will come as a major blow to Sir Bobby, who will hope that Hugo Viana will grasp his chance in the first team as effectively as the former Forest man has.