Lopes was introduced with five minutes to play and scored with one of his first touches to put City in the pot for Sunday’s FA Cup fourth round draw with a 3-0 win.
“He’s got a big smile on his face at the moment, I can tell you that,” Platt revealed in the post-match press conference.
“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.”
The 17-year old forward has been in prolific form for Attilio Lombardo’s EDS side in the first half of the 2012/13 season, so Platt wasn’t surprised at his clinical finish in the dying embers of this third round tie.
“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,” Platt declared.
“He’s got great qualities – for the under-21s he’s been scoring goals like that this season.
“He has wonderful technical ability – he’s got a good future ahead of him, certainly.”
...David Platt on Marcos Lopes...
City always looked in control of the match but were given a scare in the 25th minute when Fernando Forestieri nearly hit back 60 seconds after Tevez had given the Premier League champions a 1-0 lead.
“In the end it was straight-forward, we’re pleased with the result,” Platt said, “We didn’t get the third goal that really finished it off until right at the end.
“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.
“Costel stood up well and didn’t have a great deal to do but what he did, he did well.”
Watford manager Gianfranco Zola was philosophical in defeat, admitting that City were the better team on the day and even found time to praise Mario Balotelli for his impressive 15 minute cameo at the end of the match.
"We had our chances as I was expecting but we didn't take them as well as we normally do and they scored as I expected them to," Zola stated.
"We were playing against the champions and, even if I had my best team to choose from, we are behind them.
"I worked with Mario for Italy U21s and even though that was a short spell, I do care about him and I hope that all of the expectations of him come true because even though he came on for a short time he caused us so many problems.
"You can see all of the potential in him, but he just has to learn how to master all of that potential and I hope he does."
We’ll let you know who City will face in the fourth round of the FA Cup as soon as the draw is made on Sunday evening.