Team Ineos rider Froome, 34, was taken to hospital with a reported broken femur after crashing before stage four of the Criterium du Dauphine on Wednesday, BBC Sport reported.
"He crashed in the downhill section of the course at high speed. He hit a wall," team principal Dave Brailsford told France Televisions.
"It'll take quite a long time before he races again."
The incident took place during a practice ride on Wednesday's 26.1km time-trial course in Roanne, France.
Brailsford said, "The ambulance came quickly. He's been taken care of and waits for a helicopter to be transferred to Lyon or Saint-Etienne.
"It's a very serious accident. Clearly, he won't be at the start of the Tour de France."
Froome was eighth overall in the Criterium after three stages of the eight-day race.
He would have been chasing a record-equaling fifth victory in the Tour, which starts in Brussels on July 6.
Froome went into last year's race as favorite, holding all three Grand Tour titles, having won the Vuelta a Espana and the Giro d'Italia.
He finished third as teammate Geraint Thomas became the third Briton to win the race.