The incident was not seen by the match officials and was reviewed by the FA's panel of former referees who decided that the incident was not worthy of a red card so no action would be taken.
United manager David Moyes said after the game that he was frustrated to see Medel escape punishment for appearing to strike Fellaini in the face.
The incident occurred in the build-up to Cardiff's stoppage-time equaliser when Peter Whittingham's free-kick was headed past David de Gea by Kim Bo-kyung.
Moyes said: "If he slaps someone in the face, then you'd expect some action to be taken.
"We actually lost our concentration because we were set up for the free-kick, the referee then spends a minute or so talking to everyone and then sets it back up."
City manager Malky Mackay also admitted that the Cardiff midfielder could have been dismissed for his actions.
When asked about Medel, Mackay told Sky Sports: "He could (have been sent off) but then I also look at Fellaini and doing what he does normally and putting his head on someone else's face, and that's for me a sending off.
"But overall I'm happy that both teams deserved to have 11 men on the pitch. They didn't deserve to have any men off and neither did we."