Manchester United teenager Anthony Elanga was struck by an object during his team's Premier League game against Leeds at Elland Road on Sunday afternoon.
The incident happened after Fred restored United's 3-2 lead in the 70th minute and celebrated near the corner flag.
The object appeared to be a coin thrown by a Leeds fan.
Elanga was hurt by the object - which hit his head - and received medical attention, although he wasn't substituted as a result of the incident.
The Swede was only introduced by interim manager Ralf Rangnick three minutes before.
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Elanga then made his feeling heard to the Leeds fans in the 88th minute, scoring United's fourth goal and putting his finger to his lips. The game finished 4-2 to the visitors.
"They have made hard work of it but eventually their class has told," said United legend Gary Neville on commentary for Sky Sports.
On Elanga's goal, former England international Chris Waddle told BBC Radio 5 Live: "Another good goal. The standard of the goals has been brilliant.
"Fernandes with a little bit of skill, rolls it into Elanga's path and you'd expect him to finish that."
United and Leeds share a fierce rivalry and tensions also boiled over on the pitch, with plenty of strong tackles made throughout the afternoon.
Nine players were booked during the game, including six for Leeds.
Sunday was the first time Leeds have faced United at home in the Premier League - without Covid restrictions - since October 2003.
The last time the Red Devils played in front of a crowd at Elland Road was in September 2011, when they hammered Leeds 3-0 in the League Cup.
Discussing the rivalry between United and Leeds, Neville once said: "When we went to play at Leeds, it was brutal.
"Probably more brutal than Liverpool in some ways. The rivalry between Liverpool and United is huge, but the Leeds-Manchester United rivalry is almost like a deep hatred.
“There’s a little bit of respect between Liverpool and Manchester.
"But with Leeds, it just felt like there was a real nastiness – like they would come on the pitch if they could, it was that type of feeling."
United took the lead in the first half, with captain Harry Maguire scoring their first Premier League goal from a corner in 389 days.
Bruno Fernandes then made it 2-0 just before the interval, yet two goals in the space of two minutes from Rodrigo and Raphinha levelled the game.
The incident at Elland Road comes just two weeks after a Leicester fan was banned for life for "attacking" a group of Nottingham Forest players at the City Ground.
"I do think there has been a post-pandemic increase in antisocial behaviour and low-level nuisance disorder more broadly," football hooliganism expert Geoff Pearson told The Guardian.
Pearson went on to say: "We’ve got derbies that haven’t been played, we’ve got important cup games, these type of games are still novel experiences post-pandemic.
"So I think we will still continue to see incidents this season. I don’t think we’re out of the woods yet."
