Benitez took over at Stamford Bridge on Wednesday following the sacking of Roberto Di Matteo and reunites with the Spain international, who he previously managed at Liverpool.
Torres enjoyed a prolific spell at Anfield under the 52-year-old, but Mancini does not believe the pair have had enough time together to lead to a goal glut when the Premier League champions face the Blues on Sunday.
"I think that Fernando is a good striker," he told reporters. "Sometimes for a striker it is difficult, because if you don’t score for three or four games, for a striker it’s always difficult. I think that Chelsea are [were] a very good team with Di Matteo, they’ll be a good team with Benitez.
"They can’t change the players in two days, but they have good players, they played fantastic football until three games ago, they were on the top. For us it will be a difficult game but we want to do well on Sunday."
|6/1||Fernando Torres is 6/1 with Bet365 to score first against Manchester City on Sunday|
The Italian is, however expecting a tough task for his side at Stamford Bridge following the appointment of Benitez.
He added: "When teams change the manager, the game after for the opponent is very difficult. For us it will be more difficult, this game."
Mancini also believes the Blues remain one of the best sides in Europe, despite their potential Champions League group stage exit after the 3-0 defeat to Juventus.
"I think that Chelsea is one of the best teams in England, in Europe," he continued. "They’ve won everything in the last 10 years and today against Chelsea is always a good game and to win there could be very important for us."