Lopes, who turned 17 last week, rounded off the scoring after Carlos Tevez and Gareth Barry had put City 2-0 ahead.
Lopes is reputedly nicknamed `Rony' after the Brazilian striker, but City assistant manager Platt was quick to play it down.
Platt said: "I hope not. His nickname is Marcos and he has played five minutes in an FA Cup tie and scored a goal. He should be proud of that and equally he should be ready to train tomorrow and he will be, with the Under-21 team to play in their game on Monday.
"He's a good lad, he's got a very good temperament as well and he won't let that go to his head.
"He's got a big smile on his face at the moment. He came away with us in pre-season, did very, very well and has great lines as a footballer with great technical ability.
"He wants to hurt the opposition, wants to get in the box, and has been scoring goals like that for the Under-21 team. He's got a good future ahead of him, certainly."
Watford's best chance came with score at 1-0 when Fernando Forestieri was through one-on-one but City's keeper Costel Pantilimon, in for the rested Joe Hart, pulled off an excellent diving save.
Platt added that the second goal from Barry just before the break had been the key to the match.
He said: "The second goal really knocked the stuffing out of them and we controlled the second half in the way we wanted. If we had conceded that one-on-one chance you never know what might have happened."