Chelsea fans in the Matthew Harding stand hurled abuse and appeared to throw objects at the United players as they celebrated Javier Hernandez's winner in the 3-2 success at Stamford Bridge.
The steward, who was later taken to hospital, fell to the floor at the same time that the fans responded to the celebrations and needed approximately 10 minutes' treatment by paramedics at the side of the pitch.
"We are looking at reports of objects that may have been thrown or incidents around the steward taking a fall," a Chelsea spokesman said after the match.
"We are investigating all the incidents around that time.
"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. He's been taken to hospital."
Hernandez gestured towards the pocket of home supporters after scoring the winner for United 15 minutes from time.
His captain Patrice Evra then tried to calm him down knowing the Mexican's celebration was likely to anger the Chelsea fans.
More stewards were seen coming to the aid of their injured colleague, who was clearly in pain, before a team of paramedics ran around the edge of the pitch to help him.
After the game Chelsea cordoned off the section of the stand where the fans vented their anger at the United players.
Source: PA
Source: PA