Footage on YouTube has emerged appearing to show Manchester United midfielder Fellaini spitting near to Zabaleta's head while the City defender lay on the floor following a clash between the pair.
The FA is aware of the clip and is now considering whether it should charge Fellaini for misconduct.
It is understood referee Michael Oliver did not mention the alleged spit in his match report, but the FA can retrospectively punish Fellaini if it thinks he is guilty.
The FA will request clearer footage of the incident from broadcasters Sky before making a decision on the matter early next week.
The FA banned Hull's George Boyd for three matches after he spat at Manchester City goalkeeper Joe Hart earlier this month.