The Premier League champions have so far failed to replicate the form of last season.
Tevez was called up for international duty with Argentina earlier this month, which Mancini believes disrupted the forward's form. Silva, meanwhile, has yet to recover from his exploits with Spain at Euro 2012.
"Carlos was good for the first month then after the international break his condition was so-so," said the former Inter Milan boss.
"He worked well for one month in pre-season. He was fantastic for three or four games and played always. Then he went to Argentina for one week and maybe loses his condition.
"After the European Championship, [Silva] came back and did only 10 days for pre-season and after then went two times [on international duty] - once in Panama and once I don't know where. I think maybe he's so tired after the European Championship."
The Blues face Fulham in the Premier League this afternoon.