Lorenzo Insigne struck twice in the opening 17 minutes as Napoli claimed a 3-1 win at the Stadio Olimpico on Saturday.
Juan Manuel Vargas pulled a goal back just before the half-hour mark for Fiorentina, who were unable to find another – even after Gokhan Inler's 79th-minute red card – and Dries Mertens sealed the result for Napoli in additional time.
"I don't know why we started out tense and unrecognisable," he told Rai Sport.
"We got back into it, but it was very tough against a strong side and we were missing some key players."
"I thank the players for what they did on the night, but also the fans who cheered us on from start to finish and are continuing to reassure us now," Montella said.
"The supporters were fantastic and it wasn't easy considering the result, but they can see what the club, players and directors are bringing forward.
"Despite what some might think, the Italian league is very competitive and we'll be able to count on Rossi next season."
