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. 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.
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