Andre Silva hit a hat-trick while Hakan Calhanoglu scored and stood out, with Suso also on the scoresheet as the Rossoneri kicked off their UEFA Europa League campaign in style.
Despite the fine display, Montella believes there is still room for improvement and wants to see the players strive for it.
"We're not perfect and we've still got to look for that," he said, via UEFA.com. "It's not going to be easy to get it – if it is even possible."
Milan had been on the wrong end of a 4-1 thrashing at Lazio last weekend, but the coach was happy with the response in Vienna.
"It was weighing us down, the Lazio defeat, for the way it happened, but I was pleased with the way we approached the game today and the spirit we showed," Montella said.
"We played quickly, particularly in the first 20 minutes, finishing the game off clinically.
"We know we still have to work hard, but this was a big win for us. Now we've got to put it to one side and prepare for the Udinese game [this weekend]."
Milan's 3-5-2 formation was particularly effective - Silva and Nikola Kalinic thriving as a partnership - and Luca Antonelli believes a system that suits their new-look squad has been found.
"We analysed the defeat in Rome and looked at where we could improve. I think we gave the perfect answer tonight," Antonelli told Sky Sport Italia.
"I really like the 3-5-2 formation, but it's up to the coach how we play. All in all, though, I think we played a game that showed what we are capable of.
"Three quarters of the dressing room has changed [since last season], but I think we've built a great team and now it's up to us to demonstrate it on the pitch."