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."