After the club won their fourth Premier League crown in five years, home fans at the Etihad Stadium spilled onto the pitch in celebration.
A TV broadcast of the match appears to show a spectator making contact with the back of Olsen’s head as the Villa keeper was escorted off the field.
“Manchester City would like to sincerely apologize to Aston Villa goalkeeper Robin Olsen who was assaulted after the final whistle in today’s game when the fans entered the pitch,” reads a statement. club statement.
“The Club has launched an immediate investigation and once identified, the individual responsible will be issued an indefinite stadium ban.”
At a post-match press conference, Villa manager Steven Gerrard was asked by reporters if the club’s players had left the pitch safely.
“No is the answer to that,” Gerrard replied.
“My goalkeeper was attacked, so I think these questions should be directed to (coach) Pep (Guardiola) and Manchester City.”
According to Sky Sports News, Aston Villa said in a statement that Olsen was “absolutely well” after the incident, adding he had taken a blow to the back of the head.
The club said they were unsure the incident happened on purpose and would not press charges, Sky reported.
CNN has contacted Aston Villa and the English Football Association (FA) for comment.
Guardiola was asked about the incident after the game, saying: “I’m really sorry…I’m really sorry on behalf of the club for Olsen and Aston Villa.”
The City boss said he didn’t know the intentions behind the action, but added: “It’s always hard to understand why people react this way, how (they) can’t celebrate each other, be happy and not do that stuff.
“I don’t know how (to stop him)… You can’t put 1,000 guards or police to control there. We’re so sorry. That’s the bad news today.”
“Both have been released on bail ahead of court appearances,” a police statement said.
“Investigations into the reported assault of a player on the pitch after the final whistle are ongoing with officers working in partnership with both football clubs.”
This was after hundreds of spectators thronged Liverpool’s Goodison Park ground after Everton avoided relegation from the Premier League with a 3-2 win over Crystal Palace.