The City manager claimed Tevez's spell away with Argentina impacted on the striker's condition - even though he did not play - and said Silva's drop in form was a result of his efforts for Spain in Euro 2012.
Tevez was left on the bench against Arsenal last weekend despite being arguably City's most effective player so far this season, but he should return against Fulham today.
Mancini said: "Carlos was good for the first month then after the international break his condition was so-so. 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."
Spain midfielder Silva was one of the top players in the Barclays Premier League last season but has had a muted start to this campaign.
Mancini pointed out that, as well as being part of Spain's Euro 2012 side, he also travelled to Panama for a friendly.
"After the European Championships he 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," Mancini added.
"I think maybe he's so tired after the European Championships but I am not worried for him because even if he's not 100% he's still a good player."
Mancini also defended his decision to mix up the centre-back partnership of Vincent Kompany and Joleon Lescott, which has been the backbone of the team in the last two terms. Against Liverpool, he brought in Kolo Toure for Lescott, and against Real Madrid chose Matija Nastasic over the England defender.
"Against Liverpool it was because he had just come back from the national team and against Real Madrid I thought it was better to play in a different shape - only this," Mancini said.
"Joleon Lescott is a player like Nastasic and like Kompany and I want all the players to play well. If he deserves to play he's first choice, but this is for everyone - not only Joleon."