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.
Montella said his side, who were without Mario Gomez and Juan Cuadrado and had Giuseppe Rossi come off the bench, had left themselves too much to do after their slow start.
"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."
Montella is looking forward to having a fit Rossi at his disposal next season after another injury-interrupted campaign for the Italy international.
Fiorentina coach also praised the fans for their support on Saturday.
"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."
