Alejandro Garnacho revealed Ruben Amorim told him to go and "enjoy" his time on the pitch after being introduced at half-time in Friday night's 2-1 win over Leicester City.
United advanced to the fifth round of the FA Cup in dramatic fashion, with Harry Maguire scoring a 93rd-minute winner. He connected with a Bruno Fernandes free-kick to spark wild scenes at the Stretford End in stoppage time.
The Reds had earlier clawed themselves level via substitute Joshua Zirkzee, who cancelled out Bobby De Cordova-Reid's opener for Leicester shortly before half-time. United were booed off at the break after a dreadful first half showing.
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United were much-improved after the interval, albeit from a low base, but it was Garnacho who made all the difference after being sent on in place of debutant Patrick Dorgu ahead of the restart. And the Argentine has revealed that Amorim's half-time message for him was simple.
"He didn't have to say too much," Garnacho told MUTV. "He has said to me the things I have to do and I know what I can do.
"He said he wanted me to play my game and just enjoy [myself]."
Garnacho played a key role in creating Zirkzee's equaliser, sending over a low cross for Rasmus Hojlund, whose goal-bound effort was blocked and landed at Zirkzee's feet.
Despite keeping their hopes of retaining their FA Cup crown alive, United remain a work-in-progress under Amorim. Garnacho admitted he is enjoying life under the former Sporting Lisbon boss.
He added: "I think he is a great guy. He likes to play real football. We have to work all together and we will be a better team in the future."
