Madrid are one of the clubs to be linked with Donnarumma after he rejected an offer to extend his Milan contract beyond 2018, the Serie A club claiming their offer was worth €50 million over five years.
Cassano, who formerly played with Milan and Madrid, offered his verdict on the shot-stopper's future, insisting a switch to Santiago Bernabeu is too good to turn down even if San Siro is the best place for him.
"It would be good for Italian football if he remained in Serie A. But it is true that if Real Madrid are there, you cannot say no."
Meanwhile, Cassano believes his former Italy team-mate Mario Balotelli, who he previously described as overrated, has found his level at Nice.
Reports in France suggest free agent Balotelli – like Donnarumma a client of Mino Raiola – is close to signing a new contract that will extend his stay with the Ligue 1 club beyond the 2016-17 season and Cassano feels that would be a wise approach.
He said: "Balotelli had a good career and he is fortunate to have Mino Raiola as an agent to help him earn a lot of money. I think Nice is the size of club he belongs at."