Lance Armstrong says Astana team-mate Alberto Contador still has "a lot to learn" about racing.
Armstrong made his comments to French newspaper L'Equipe following Contador's fourth-place finish in the Paris-Nice.
The Spaniard was leading the event until losing three minutes to eventual winner Luis Leon Sanchez on the penultimate day.
Contador, the Tour de France winner in 2007, admitted afterwards he had run out of energy after forgetting to eat in the excitement of what was a gruelling mountain stage.
"Contador has a lot to learn," Armstrong said in Tuesday's edition of L'Equipe.
But the American was not all critical of his young rival, adding: "He's the best in the world. His only negative trait, and I say this with much respect, it's that he's too nervous. He's too strong to be so nervous."
Armstrong, 37, returned to professional cycling at the start of the season after three years in retirement.
The Texan is bidding to raise awareness in the global fight against cancer, a disease he survived prior to winning the Tour de France seven times in a row from 1999.
He plans to ride in both the Giro d'Italia and alongside Contador in the Tour de France later in the summer.
The high-profile pair will race together in Astana colours for the first time at next week's Tour of Castilla y Leon in Spain.