The Portuguese youngster came off the bench to score City's third goal in a comfortable win over Championship outfit Watford.
Lopes found the net in injury-time when he reacted quickest to fire home after Watford goalkeeper Jonathan Bond could only parry Mario Balotelli's shot.
"He has got a big smile on his face at the moment [after scoring]," said Platt. "He came away with us in pre-season and did really well.
"He has great technical ability and for the under 21 team he has been scoring a lot of goals like that.
"He has got a good future ahead of him certainly."
Platt was happy with the manner of City's win over Watford as first half goals from Carlos Tevez and Gareth Barry put them in control before Lopes put the icing on the cake.
"In the end the third goal really finished it off, but I thought the two goals took the stuffing out of them and controlled the way we played," added Platt.
"If we had conceded the one-on-one chance it might have been different, but Costel [Pantilimon] stood up well.
"He didn't have a lot to do, but what he did, he did well."