The Italian side thrashed Austria Wien 5-1 for a second time this season, with strike duo Andre Silva and Patrick Cutrone both scoring twice in the San Siro rout.
The result, coupled with AEK Athens' 2-2 draw with Rijeka, means Milan have secured top spot in Group D with a game to spare.
But while Montella's side have impressed in Europe they are languishing down in seventh place in Serie A, a situation their manager hopes they can now rectify in the coming weeks.
"It was an extremely positive evening and we will honour the final game in the Europa League group phase, as that is what our history demands," he told Sky Sport Italia.
"Qualifying top of the group was the minimum objective. We achieved it, now we turn our focus to making up lost ground in Serie A.
As for the visitors, Wien boss Thorsten Fink felt his side fell apart after taking the lead, albeit the result has not completely ended their hopes of joining Milan in the knockout stages.
The Austrian team can qualify courtesy of a better head-to-head record if they defeat AEK Athens in the final round of group fixtures.
"Our plan worked for 20 minutes. After we scored the goal, we somehow lost our confidence," Fink told UEFA.com.
"We didn't attack anymore and we defended horribly. Anyway, I'm happy that our hopes are still alive."