Analyze these things, did you change formations. Did you change his main position he plays. Are you still winning normally or same as other seasons. The game usually chooses the players that are gonna be the main scorers and assist providers before hand (how I cannot explain, I just observe). As long as you are winning don't worry he is doing his job even if it may not be reflected. Don't even think about selling him.

Try this if you are concerned, use your reserve instead of him in 1 or 2 matches with the same formation and tactics and see if it makes a difference to the overall team performance. This may be risky but maybe it will clear your doubt.