Alejandro Garnacho clashed with a fan on social media after Man United's 3-0 defeat by Liverpool on Sunday.
Garnacho, 20, established himself as a first team regular at Old Trafford last season but has so far been eased into action this time around following his exploits at the Copa America with Argentina over the summer.
The winger made his first start of the campaign against the Reds at the weekend, but struggled to impact the game, before he was replaced by Amad Diallo midway through the second half.