The 25-year-old has been strongly linked with a summer move to Barcelona but now Migliaccio has thrown United's hat into the ring.
With rumours of Ronaldo's proposed move to Real Madrid refusing to go away, the former Marseille man's representative believes that, should the Portugal international join the Spanish champions, United would look to replace him with Ribery.
The French international star has been in scintillating form of late, highlighted in Bayern's 5-0 demolition away at Sporting Lisbon in the UEFA Champions last-16 first leg where he scored twice and created two more.
"If Manchester United sells Cristiano Ronaldo, Sir Alex Ferguson would like to cover that transfer with Ribery's signature," Migliaccio told Bild.
Meanwhile, Barcelona president Joan Laporta denied that his club had already reached an agreement to sign Ribery at the end of the season.
"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."