Mancini hooked Silva on the hour mark for their 3-3 draw with Sunderland.
"I don't know how many games David has played for us in the Premier League, Champions League, Europa League, Carling Cup, FA Cup plus internationals," said Mancini.
"But it's normal that a player can arrive at a moment when he can't play 100 per cent."
"David always plays here [indicates high level with hand] and now maybe he is here [drops hand lower to chest]," he added. "This is normal, but I hope he can recover."