Spanish cyclist Robert Heras has signed a three-year contract with Spanish-based outfit Liberty after quitting US Postal last week.
Heras is a mountain specialist
Heras was one of Lance Armstrong's most influential team-mates as the American won the last three Tours de France.
US Postal have signed up Portugal's Jose Azevedo to replace Heras.
The Spaniard had a year of his contract left to run and Liberty had to pay a £1.1m get-out clause.
Armstrong said on Friday: "I think it's logical he wants to go somewhere and lead a team. It makes sense.
"There's no hard feelings and it will be good for us to have him competing against us. It will keep the race tempo high."
US Postal team manager Johan Bruyneel had admitted last week: "Nobody is irreplaceable but we wouldn't like to lose him."
Heras played a key role in shepherding the American through the mountains on his way to a fifth straight victory earlier this year.
He confirmed his status as one of the world's best climbers with a brilliant performance on his way to victory in this year's Vuelta.
Some believe he is one of the few riders capable of challenging Armstrong in the Tour.