City blew a two-goal lead in the second half before Mario Balotelli slotted home a penalty to seal the win.
"We should have put the game to bed at 2-0. That is what the manager said afterwards," Platt said.
"His voice was low but what he said was profound. He is not happy at the gift we gave them for the first goal and which let them back in.
"It was a lapse in concentration, we gifted it to them and the manager has made that clear to the players in no uncertain terms."
City lead the Premier League with 54 points.