Both sides head into the encounter at San Siro having progressed to the last 16 of the Europa League on Thursday.
Inter beat Celtic 1-0 at home on Thursday, courtesy of Fredy Guarin’s goal, for a 4-3 aggregate victory while Fiorentina ousted Tottenham 3-1 on aggregate with a 2-0 triumph in Florence. They next face Wolfsburg and Roma respectively.
“We deserved to win against Celtic and we are happy to be in the next round,” Mancini told his club’s official website.
“We are growing but we are not perfect yet. We are still making mistakes but we are definitely on the right track.”
The Nerazzurri won 2-1 at Cagliari on Monday for their third straight league win – the first time they have achieved that feat since November 2012 – to climb to eighth in Serie A.
Mancini believes his side faces a huge hurdle on Sunday to extend their good run of form.
“It’s going to be tough,” he said. “We need to recover all of our energy.
“Fiorentina are playing well and deserve to be in the position they are in. In order to beat them, we will have to play a great game.”
Fiorentina are unbeaten since January 6 and last weekend’s 1-1 draw against Torino lifted the Viola to fifth place, four points clear of Sunday’s rivals.
Viola coach Vincenzo Montella was delighted with his team’s performance against Tottenham and expects another strong showing from his side this weekend.
“Against Tottenham, the team proved they are convinced of their strength,” Montella said. “The important thing is to play with enthusiasm and not with euphoria.
“It’s right that our fans have big dreams but we are professional and from tomorrow, my team will focus on another good performance at Inter.”
Thursday’s goalscorers Mohamed Salah and Mario Gomez are set to lead Fiorentina’s attack.