Manchester United starlet Anthony Elanga has been struck on the head by an object thrown from the Elland Road crowd this afternoon.
In a topsy-turvy meeting with their bitter rivals from across the M62, it was United who coped better with testing conditions during an eye-catching 4-2 victory.
Goals from Harry Maguire and Bruno Fernandes put the Old Trafford giants in control.
However, the hosts responded upon the restart and restored parity through Rodrigo and Raphinha.
A quick-fire double had threatened to destabilise United after what has been a relatively comfortable opening 45 minutes for the top-four challengers - but Ralf Rangnick started to make changes and United went on to seize the initiative, once again with substitute Fred firing them back in front.
United celebrated that strike in front of the Leeds supporters, but there was an unsavoury flashpoint when Elanga, who was also summoned from the bench and proceeded to add a fourth goal late on during another lively cameo, went down clutching his head.
The teenager appeared to have been struck by a blue object that was thrown from the stands.
An isolated incident as these two clubs locked horns in front of fans for the first time at Elland Road since 2003 in the Premier League, match-goers had been given a warning about their behaviour pre-match.
In his programme notes, Leeds chief executive Angus Kinnear said: "While the hostility of Elland Road is a definite competitive advantage we would all encourage, let's not mistake it for unsavoury and illegal behaviour that could mar our reputation and it is not befitting of our club or city."
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