A Manchester City fan appeared to throw an object at Manchester United star Fred during the derby at the Etihad.
The home side were being beaten 2-0 at the time when Fred had gone to take a corner and was hit by an object.
The Brazilian held his head as City players appealed with fans to calm down.
The objects appeared to be a plastic bottle and a lighter in the incident in the 70th minute.
City midfielder Fernandinho was seen remonstrating with the supporters, urging them to behave as his side tried to get back into the game.
Referee Anthony Taylor eventually waved play to continue with United leading 2-0.
Marcus Rashford scored the opener from the penalty spot after being fouled by Bernardo Silva.
Anthony Martial added a quickfire second for the visitors before Ilkay Gundogan got one back for the home side.
City went into the clash 14 points adrift of Liverpool at the top of the table following their comfortable win over Bournemouth earlier in the day.
Fred was a target for Manchester City before he made a £53m move to Old Trafford in 2018.
He endured a difficult first season but has been in much better form since returning to the side this term, since the injury to Paul Pogba.
Pogba was in the stands at the Etihad but Ole Gunnar Solskjaer admits he is still a few weeks away from returning to the fold.
Earlier this week, Rangers captain James Tavernier was hit by a pie thrown at him during his side's 2-2 draw with Aberdeen.
