United midfielder Fellaini and striker Aguero clashed in the first half of City's 1-0 victory at the Etihad Stadium on Sunday.
Aguero tumbled to the floor after Fellaini kicked his standing leg and the hosts appealed for a penalty, which was not given.
Fellaini clearly did not feel it was a foul and he bent down to shout at the Argentinian as he lay face down on the floor.
TV replays showed that some spit appeared to exit the Belgian's mouth as he unleashed his tirade at the striker, but Fellaini insists he did nothing wrong.
"The press loves to invent the stories when there aren't any," he tweeted.
"Thanks the journalist
#ferity #tospit."
The Football Association will wait to see if referee Michael Oliver mentions the incident in his report before deciding whether to take any action against Fellaini.
If the alleged spit was not seen by the referee or any of his assistants, the FA video review panel could take disciplinary action against the Belgian if they think it was a deliberate act.
United manager Louis van Gaal defended his player after the match.
'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,'' the United boss told a press conference.
''I don't think that he's a spitting figure.''
Van Gaal's opposite number Manuel Pellegrini said he was not aware of the incident.
Source : PA
Source: PA