The 17-year old pounced on Jonathan Bond's parry of Mario Balotelli's late effort to tuck home his first for the club and City's third just minutes after coming on for his debut.
David Platt: "He's got a big smile on his face at the moment, I can tell you that.
"Marcos has played five minutes at the end of an FA Cup tie and he's scored a goal.
"He should be proud of what he's done but, equally, he should be up tomorrow and ready to train hard which he will be because he's got a good temperament and he won’t let this go to his head.
"He came away with us on pre-season and did very, very well, he's a very technical footballer, you can see that he wants to go and hurt the opposition.
"He's got great qualities - for the Under-21s he's been scoring goals like that this season.
"In the end it was straight-forward, we’re pleased with the result.
"That second goal just before half-time really takes the stuffing out of them and that second half we controlled the way the game was going to be played.
"Goals change games. We went 1-0 up and if we had conceded straight away when they had the one-on-one chance, you never know how it might have gone."