Ralf Rangnick condemned the crowd after Leeds' clash with Manchester United following Anthony Elanga being struck by a coin.
The Red Devils picked up a thrilling 4-2 victory in a fiercely competitive contest at Elland Road, with Harry Maguire, Bruno Fernandes, Fred and Elanga all getting on the scoresheet.
The game was overshadowed by a group of Leeds supporters though, as Elanga had to be treated on the pitch after being hit by an object from the stands.
What happened?
Members of the home crowd were clearly seen throwing missiles after United's first and second goals.
Fred made it 3-2 after coming off the bench in the second half, and fellow substitute Elanga was hurt by a coin to the head as he joined in the celebrations.
The 19-year-old was able to finish the match despite the blow, and had the final say by scoring United's fourth goal before running to the away stand and putting his finger to his lips.
What has been said?
The Press Association have confirmed that the FA will be launching an investigation into the incident, and Rangnick said Leeds had already started their own internal process when speaking to reporters after the game.
"As far as I know, there is some investigation going on from Leeds United," said United's interim manager.
"I don't think that anything happened in the end. Anthony Elanga looked well and he was happy about scoring his goal.
"Obviously things like that should not happen and I think even more so this game. The atmosphere was great.
"At times we had to cool down things on the pitch a little bit but this is what I did when I walked onto the pitch (during a late melee). I didn't want anything to happen - no yellow or red cards. In moments like this, I think it's important to try and cool things down."
Nine arrests after Pennines derby
Police made a total of nine arrests after the final whistle at Elland Road after a number of other "minor incidents" during the game.
Deputy Chief Constable Russ Foster of the West Yorkshire Police has said in an official statement: "High profile football matches, such as yesterday’s fixture can clearly attract some of the greatest concentrations of people and there were a few incidents both before and during the game.
“We had a large group of 70 people who turned up in Leeds without tickets and they were swiftly identified and issued section 35 dispersal notices before the game and were not allowed to enter the grounds. We also made a handful of arrests for a number of offences including public order, throwing of missiles and breach of banning order.
“Anyone with any information about any incidents or footage from the day, is asked to contact the police via 101 or online."