The steward was hospitalised with a twisted knee, having injured himself after slipping on the turf following Javier Hernandez's winning goal in Manchester United's 3-2 victory at Chelsea's home ground.

Hernandez celebrated in front of the Chelsea supporters, who reacted angrily, and the steward was involved in ensuring the Manchester United striker's safety before falling to the ground and requiring treatment, which is when he allegedly was struck by a thrown object.

"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 Sunday's game.

"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's goal came after both Branislav Ivanovic and, more controversially, Fernando Torres were sent off for Chelsea as they crashed to their first defeat of the Premier League season so far.

The loss reduced their lead at the top of the table to only one point from Manchester United and Manchester City.