Ribery was lately linked with a move to Barcelona in the upcoming transfer window, but his agent revealed that United might be the France ace's next destination.
"If Manchester United sells Cristiano Ronaldo, Sir Alex Ferguson would like to cover that transfer with Ribery's signature," Migliaccio told German newspaper Bild.
Spanish champions Real are still believed to be interested in Ronaldo's service, despite the rejection of their previous overtures.
Meanwhile, Barcelona chairman Joan Laporta refuted reports claiming that his club had reached an agreement with Ribery over a summer deal.
"We didn't speak with his representative nor with the player's entourage," he told L'Equipe.
"If one day we were to plan it, we would speak directly with his club."