Inter are just four points adrift of fifth-placed Fiorentina in eighth following run three straight victories in the league.
Victory over the Florence club would be a significant boost to Inter's prospects of European qualification, however, Mancini believes Fiorentina are better in the final third.
"Fiorentina play some lovely football whereas I think we need a bit more time and a bit more quality," Mancini said.
"Fiorentina have very technical players and we must be concentrated. They fixed some issues at the back, but still have an attacking mentality. They play well and have been working together for a number of years."
Speaking about Montella - who he played with at Sampdoria - Mancini added: "He scored many goals thanks to me. It was a real pleasure to play alongside him.
"He's a good lad and deserves all his current success. He makes Fiorentina play really good football.
"Montella has enough experience to be a coach at any club, even the biggest ones, but a young tactician has to think carefully about his future and choose the side that will allow him to work in the best circumstances."
