Gianluca Lapadula gave the Rossoneri the lead after just 15 minutes of the match at the Stadio Carlo Castellani, but Riccardo Saponara levelled things up for the hosts before the break.
Second-half goals from Suso, Lapadula and an own-goal from Andrea Costa handed Milan the victory, but after the match Montella conceded that he was far from pleased with his side's performance during the opening 45 minutes.
“The way I saw it was we struggled more than necessary in the first half. We were too negative, too lazy, didn’t trust ourselves and the preventative marking was just awful,” the Coach told Sky Sport Italia.
Montella was pleased with the way his side responded during the second-half, and the victory means Milan are now up to second in the Serie A standings.
He added: "I liked the second half much more and we created chances from the back, proving that we are capable of doing that even with other players."