Milan relied on penalties in either half from recruits Ricardo Rodriguez and Franck Kessie to see off a stubborn SPAL side, who offered next to no attacking threat of their own at San Siro midweek.
Montella, however, was quick to point out that Milan have played four games in 10 days, and recognises that performances like this can happen when players are tired.
"A mature team understands that we are at the fourth game in a few days and was to be treated differently by the players," he told Milan TV.
"It is a precious and well-deserved victory, but we have to play with more continuity for the 90 minutes.
"It was not easy as SPAL played with great order, but the team were always in the game and know what to do, even in defence."