The Belgium international bent over to shout at Aguero after the striker went to ground appealing for a penalty in the 39th minute of the clash at the Etihad Stadium, which City won 1-0 thanks to the Argentina international's second-half goal.
While referee Michael Oliver decided not to award a penalty, the incident drew an angry response from Fellaini and there was a strong debate on social media as to whether he spat at Aguero.
United manager Louis van Gaal was quick to absolve his player of blame, stating after the game that "sometimes when you shout there's a little bit of saliva [that comes out] with it".
Fellaini has now also responded to the claims, suggesting that it is merely the media trying to create an incident from nothing.
"The press loves to invent the stories when there aren't any," he wrote on Twitter. "Thanks [to] the journalist."
Fellaini was involved in a similar incident during last season's Manchester derby at Old Trafford, when he was alleged to have spat at Pablo Zabaleta, who was also laid on the turf, though no action was taken against him.
