The incident occurred during Tuesday's Manchester derby at Old Trafford after the Belgian had fouled the City defender.
Fellaini was booked by referee Michael Oliver for elbowing Zabaleta half an hour into the match.
Video footage then shows the Belgian turning away from Oliver as the referee gestures for the physio to come onto the pitch and attend to Zabaleta.
The Manchester United midfielder walks past Zabaleta on the floor and appears to spit in his direction.
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