Match footage has emerged appearing to show Manchester United midfielder Fellaini spitting near to Zabaleta's head while the City defender is laying on the floor following a clash between the pair.
The Belgium midfielder had been yellow carded for a nasty elbow on the Argentina full-back.
The Football Association is aware of the clip and is now considering whether it should charge Fellaini for misconduct.
The FA will request clearer footage of the incident from broadcasters Sky before making a decision on the matter early next week.
David Moyes has discussed the claim with Fellaini and says the Belgian is adamant he did nothing wrong.
"I've spoke with Marouane and he says it isn't the case," the United manager said.
"We will work with the FA if that is what we have to do. Marouane is adamant that at no time did he do that (spit at Zabaleta). I don't think it is completely conclusive, from what I've seen."
The FA banned Hull's George Boyd for three matches after he spat at Manchester City goalkeeper Joe Hart earlier this month.
Tuesday's referee Michael Oliver booked Fellaini moments before the alleged spit towards Zabaleta for the elbow.
After the 3-0 win for City, Zabaleta claimed Fellaini was lucky to stay on the pitch.
"I was elbowed by Fellaini. It was a red card," he said.