Valentino Rossi's hopes of claiming a seventh MotoGP world title are all but over after he suffered a fractured shin at Mugello on Saturday which could rule him out for up to three months.
The Fiat Yamaha rider, who sits second in the MotoGP riders' standings ahead of Sunday's Italian Grand Prix, crashed heavily at turn 13 with 15 minutes remaining in the practice session.
Dr Claudio Costa of Clinica Mobile, MotoGP's travelling medical facility, revealed Rossi could be sidelined until September by the "very serious" injury, which would rule him out of at least eight races, starting with Sunday's Italian Grand Prix.
"It's a more serious fracture than we expected," Costa told reporters.
"It's a compound fracture which involves both the tibia and the fibula. I hope there are no complications."
"It will be at least two or three months for him to recover. It's not the usual break of the bone, it's very serious."
The Italian, who has won a total of nine grand prix world championships, was removed to the Tuscan circuit's medical centre for X-rays, where the extent of the reigning champion's injuries became clearer.
Yamaha team manager Davide Brivio revealed Rossi would undergo an operation on his right leg.
"We have decided for him to be operated on immediately, but we won't rush his recovery time. It will be a normal recovery," said Brivio.
Yamaha confirmed Rossi has been taken by helicopter to the Careggi Hospital in Florence before moving to a nearby specialist.
Rossi was eyeing a 10th career victory at his home circuit this weekend, and seeking to end the strong sequence of performances by team-mate Jorge Lorenzo, who has won the last two races to move nine points clear at the top of the championship standings.