Balotelli played his first European game in a Milan jersey, as when he arrived in January he was cup-tied.
Montolivo said: "Since Mario arrived in January he has been incredibly consistent in terms of goals and performance, so of course he's important for us.
"Compared to last season we are at a more advanced stage, as the players already know each other. We need to improve match fitness, but that's only to be expected.
"I think it is an advantage that we have more or less the same side as last season. This time last year we were a work in progress, so we hope this time to avoid that rough start."
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