The Football Association is investigating after Manchester United's Anthony Elanga was struck by an object thrown from the crowd in Sunday's Premier League victory at Leeds United.
There were also numerous unsavoury chants from both sets of supporters.
It was the first game in front of fans between the two old rivals at Elland Road since October 2003.
Elanga scored his side's fourth goal with two minutes remaining to confirm victory for Ralf Rangnick's visitors, after Leeds had levelled from two goals down in a fiery encounter.
Arrests at football matches across the top five English leagues are at their highest levels in years, with fan disorder "getting worse", according to the UK's football policing lead.