Marseille president Christophe Bouchet told the club's website that both clubs and the player had agreed a loan until the end of the season, but the move has yet to be approved by soccer's world governing body FIFA.
"It was in everybody's interest," said Bouchet.
"It was in Marseille's interest because it is necessary to rely on two great goalkeepers when you're ambitious.
"It was in Fabien's interest because he will be able to prepare for Euro 2004 and play. It was in Manchester's interest because he was longer playing," Bouchet added.
The 32-year-old, who won the 1998 World Cup and Euro 2000 with France and remains number one choice for his country, has slipped to third in the pecking order at English champions United after being usurped by U.S. keeper Tim Howard and second-choice Roy Caroll.
France coach Jacques Santini made it clear earlier this month that, unless Barthez played regularly, his six-year reign as France keeper may be nearing its end.
Marseille need a keeper following an injury to their number two keeper Cedric Carasso last week.
"He's a born fighter. And he played long enough at the Velodrome to know that it's not a pitch that you walk about on with hands in your pockets," Bouchet added.
RUNJE SAFE
Barthez started his professional career at Toulouse and joined Marseille in 1992, winning the European Cup the following year. He later moved to Monaco before joining Manchester United in 2000 for 7.8 million pounds ($13 million).
Bouchet said Barthez's arrival would not be at the expense of current first choice goalkeeper, Croatia's Vedran Runje.
"Marseille's first goalkeeper is Vedran Runje and this won't change. But to have Barthez with us on loan is also a plus for Vedran.
"To play alongside a keeper who has been at the top for ten years will help Vedran make it to the very best in the world," he said.
Runje himself says he is not worried about losing his place to the World Cup winner.
"He will have to show he is better than me," the Croatian keeper told reporters on Thursday. "I didn't come to Marseille to sit on the bench. I am not going to be frightened even if I respect him a lot."
