Aston Villa goalkeeper Robin Olsen was attacked during the pitch invasion at the Etihad Stadium following Manchester City's dramatic title win.
Olsen was struck on the back of the head as he attempted to leave the pitch at the final whistle following City's dramatic 3-2 win.
Thousands of City fans charged onto the pitch after the final whistle, with players caught up in the mayhem and unable to reach the tunnel safely.
Villa boss Steven Gerrard said: "Our goalkeeper was attacked.
“We're going to see how he is now, but I think you should ask Pep [Guardiola] and Manchester City those questions."
City said in a statement: "Manchester City would like to sincerely apologise to Aston Villa goalkeeper Robin Olsen, who was assaulted after the final whistle at today’s match when fans entered the pitch.
"The club has launched an immediate investigation and once identified, the individual responsible will be issued with an indefinite stadium ban."
The attack is the latest incident to blight the game following a spate of pitch invasions in recent weeks.
A Nottingham Forest fan was jailed for six months last week for attacking Sheffield United's Billy Sharp at the City Ground.
A pitch invasion at Everton on Thursday saw Crystal Palace boss Patrick Vieira kick out at an Everton fan who taunted him.
Stand-in stopper Olsen was making his first appearance for City in place of usual No.1 Emi Martinez, and he and fellow Villa teammates were quickly forced off the pitch as City fans celebrated.
Liverpool eventually ran out 3-1 winners over Wolves at Anfield, but City's victory saw them finish on 93 compared to the Reds' 92.
City double goal hero Ilkay Gundogan said: "It was an unbelievable game. It was fantastic. Honestly, after going 2-0 down, the chances were very small. We had to do simple things again in the best possible way.
"It was more of a negative one than a positive one. It was about getting that goal and we knew it would be one our side to be able to score three goals. We’re fans at the end of the day. These are days that you look back too.
On Liverpool, he added: "It’s tough to say, if Liverpool would not be there and play the way they do, I don’t think the league could be that attractive.
"They played an incredible season and we pushed each other to the limit. Even though it's a sad day for them, we need to appreciate what they’ve done. We’re looking forward to competing with them against next season."
