During the course of a typially fiery clash between the local rivals, which Premier League champions City evantually won 1-0,Fellaini vented his anger at Aguero after the City striker appealed for a penalty towards the end for the first half.
Aguero went to ground after Fellaini kicked his standing leg but on spot-kick was forthcoming.
Belgian international midfielder-Fellaini was unimpressed with Aguero and shouted at the Argentina striker while the forward was down on the truf.
According to some observers, spit left Fellaini's mouth in the midst of his verbal tirade but United manager van gaal insisted it wasn't an international act buy his player.''The TV has shown that he is shouting and sometime when you shout there's a little bit of saliva {that comes out} with it,'' van Gaal said.
''I don't think that he's a spitting figure,'' the Dutchmen added.
Fellaini endured a poor matah at the Etihad stdium, missing a chance to equalise with a poorly directed header after the impressive Aguero scored the only goal of the game when he turned in Gael Clichy's cross in the 63rd minute.