Andy Robertson was left red-faced after his failed attempt to put Mason Greenwood off by screaming in his ear as the Manchester United striker scored a first-half equaliser was spotted by fans.
Paul Pogba broke up a Liverpool attack and Donny van de Beek sent a quick pass on to Marcus Rashford, who darted down the left before cutting back and sending a fine a diagonal ball over to Greenwood.
The teenager fired a low right-footed strike past Alisson in the 26th minute.
Mason Greenwood held off Andy Robertson to fire Manchester United level at Old Trafford
And TV replays showed that Robertson appeared to shout at Greenwood as he tried to close him down in an attempt to put him off.