The Viola claimed a 4-3 win over Hellas Verona on Monday evening, but Montella felt that they should have scored more goals at the Stadio Artemio Franchi.
"We should be more ruthless in defence and attack," he told Sky Sport Italia.
"I have to give praise to our opponents but we cannot always score four times to secure a victory.
"In this game we had so many chances to extend our advantage going forward and at the back we have got to maintain our concentration better."
Fiorentina are currently fifth in the Serie A table, level on 27 points with Inter Milan.