The Parma keeper is in line to replace the Old Trafford star at both international and club level in the future, providing Sir Alex Ferguson can set up a deal to sell Barthez.
United are discovering that it will not be easy to find a buyer for their current number one, despite offering him to several clubs in France, and Barthez himself has already insisted he is happy to remain in Manchester.
The 31-year-old was axed from United's title run-in, after an unimpressive season culminated in below-par performances in the Champions League quarter final matches against Real Madrid.
Planetfootball.com understand The Red Devils are keen to sign Frey, also a target for Arsenal, but their efforts are dependent upon the sale of his fellow Frenchman.
"I can confirm United are very interested in Sebastien," Frey's agent Claudio Vigorelli exclusively told planetfootball.com.
"But there are only a few possibilities that my charge will go to Old Trafford.
"He would be very happy to play for such a big team, but that would only happen if Barthez goes."
United may have managed to ship midfielder David Beckham to Real Madrid, seemingly at least a little against his wishes, but stubborn World Cup winner Barthez could prove a different proposition with only a less glamorous employer on the horizon.
"It will not be easy for United to find someone interested in Barthez," stressed Vigorelli.
"Where can he go?
"I don't think he has requests from any Premiership club, and I'm sure he has none in Italy or Spain.
"He could only go back to Le Championnat.
"Should United find a solution for Barthez, then Sebastien would be a more than serious candidate to replace him."
