During the course of a typically fiery clash between the local rivals, which Premier League champions City eventually won 1-0, Fellaini vented his anger at Aguero after the City striker appealed for a penalty towards the end of the first half.
Aguero went to ground after Fellaini kicked his standing leg but no 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 turf.
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 intentional act by his player.
“The TV has shown that he is shouting and sometimes 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 Dutchman added.
Fellaini endured a poor match at the Etihad Stadium, 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.