Donadoni believes the former Real Madrid midfielder can make a big difference for the Rossonero.
He told La Gazzetta dello Sport: "He has such knowledge of the game, experience, and personality that he can make the difference in Serie A even at nearly 40. Age can obviously affect the amount of time he plays, but not the quality: we saw that the other night in the Coppa Italia against Bari.
"And then we shouldn't forget that AC Milan are without any major competition; Modric will have a week to recover between matches."
On Milan's summer market, Donadoni also said: "From (Ardon) Jashari to (Pervis) Estupinan and (Koni) De Winter, AC Milan have chosen interesting players, but they must prove themselves at the highest level. I think they all have the quality to make the leap, but I'm also waiting to see who arrives in attack before making a final assessment."