Javier Hernandez appeared to be pelted as he celebrated scoring United's winner in front of the Blues fans in the Matthew Harding Stand.
After that, a steward required 10 minutes of treatment before being taken to hospital.
"We are investigating all the incidents around that time," said a spokesman.
"We are looking at reports of objects that may have been thrown or incidents around the steward taking a fall.
"My information so far is that he slipped and fell, may have hurt his knee, and was being treated by the side of the pitch."