The 18-year-old has opted against signing a contract renewal, with his existing deal due to expire in 2018, meaning Milan are likely to cash in during this transfer window to prevent him from leaving for free next year.
Juventus have yet to table an offer for Donnarumma, but general manager Marotta has made it clear they are ready to pounce if Milan give the green light for a transfer.
"It is becoming increasingly difficult for clubs to handle these situations. We are facing a change in behaviour when it comes to young players," Marotta told Corriere della Sera.
"Right now, we are not after Donnarumma.
"Having said that, the situation is dictated by the rules and we have a right to examine all options on the transfer market.
"I hope both parties reach an agreement, but if a player like Donnarumma ends up on the market, we have a duty to try to get him."
Juventus have also been linked with Marco Verratti in recent weeks, with the midfielder allegedly keen to leave Paris Saint-Germain, but Marotta is adamant the Serie A champions have no plans to spend €100 million on the Italy international.
"A transfer like that of [Gonzalo] Higuain last season was extraordinary," Marotta added.
"Right now, you see some crazy figures being mentioned on the transfer market.
"When you look at the tactical situation and the amount of money he would cost, we are not interested in Verratti. We are not going to spend €100 million ($147M) on him."