The Chelsea midfielder has agreed an 18-month contract at the San Siro and is Seedorf's first signing since taking charge.
"I know him very well", Seedorf said of the 31-year-old.
"He's a great player, a warrior I think to use in the midfield.
"He will give also bring big values on and off the pitch. He's a guy with personality that will be useful for the team."
Seedorf also spoke about the difficulties Japanese star Keishuke Honda has experienced in his first few weeks in Serie A, after he joined on a free transfer.
"I have followed him for are many years, we must be patient with him. It's not easy passing from Russia to Serie A. We must give him time and we must help him with Italian football."