The 33-year-old Dutchman was a key component of the Milan sides that knocked United out of Europe in 2004-05 and 2006-07.
Rooney said: "Seedorf. He is probably the strongest player I have ever played against. I have rarely seen someone do what he does on the pitch. You can't keep him out.
"And then there is Pirlo and Beckham: Milan are a really great team. Playing at the San Siro is special."
Rooney also had praise for Ronaldinho.
"He is incredible. Ronie has a fantastic talent. He can resolve any game in any moment on free-kicks," Rooney concluded.