Kaka, 31, ended a disappointing four-year stay with Real Madrid in the summer when he re-joined Milan – the team that he departed in 2009.
"You have to be patient with Kaka. I hope that the fans do not expect to see the Kaka from the past right away," Gattuso told Sky Sport Italia.
"We should not forget that he has not been playing much for the past three years and has been unable to act at his highest level.
"That's why the Milan fans should be patient and give him time. Kaka is a great player and also a good person to have in the dressing room."
Kaka made the move to Italy on a free transfer.