Bonucci made the surprise switch from Serie A champions Juve to the San Siro last week after the two clubs agreed a reported €40million fee for the Italy international.
The 30-year-old is one of 10 new faces at Milan ahead of the 2017-18 campaign as Montella looks to guide the club back into the Champions League for the first time since 2014.
In recent seasons Bonucci has played in a back three with Giorgio Chiellini and Andrea Barzagli for Juve and Italy and Montella could be willing to adopt the same tactic.
"I am evaluating the players that I have available on a daily basis," he said after Milan's International Champions Cup defeat to Borussia Dortmund on Tuesday.
"I have so many players, I think they're capable of offering different tactical solutions.
"Bonucci's arrival, for example, could see me favour a three-man defence, but it's difficult to know for sure right now."
Milan were without Bonucci for the 3-1 defeat to Dortmund, but he could make his first appearance for the Rossoneri against Bayern Munich on Saturday.