Fabio Borini completed AC Milan's triumph over Ludogorets Razgrad, with a 1-0 home victory capping off a 4-0 aggregate win.
Borini scored following a counter-attack in the 21st minute, turning in a Patrick Cutrone cross at the far post.
"I am happy we kept a clean sheet, as that's something I was focusing on in training lately," coach Gennaro Gattuso told Sky Sport Italia.
"It's good that nobody had a cramp after 90 minutes, but it's about attitude too. I am very, very happy with the growth of the team and those who haven't played as much and ensured they were ready when called."
Borini told reporters after the match he wants to put pressure on his boss to include him in the squad.
"We knew Ludogorets would start very strong, it was important not to concede and we also scored a goal that gave us more confidence," the versatile forward told Sky Sport Italia.
"I do everything to make it difficult for the coach to leave me out, then we'll see what happens."