The steward was hospitalised with a twisted knee, having needed treatment after the home supporters had reacted angrily to Chicharito, who scored the winner, celebrating in front of them.
"We are looking at reports of objects that may have been thrown or incidents around the steward taking a fall," said a Chelsea spokesman.
"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."
The Mexican's winner 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, a loss that closed their lead at the top of the table to only one point from second-placed Manchester United.