Valentino Rossi has undergone an operation to pin broken bones, suffered in Thursday's enduro training accident in Italy.
The nine-time world champion suffered displaced fractures to the tibia and fibula of his right leg in the accident near his hometown of Tavullia.
No time frame has been put on Rossi's return but the Movistar Yamaha rider has vowed to be back on a bike as soon as possible.
He was taken to the Ospedale Civile di Urbino hospital for tests, where the extent of his injuries was confirmed.
The 38-year-old Italian was transferred to Ospedali Riuniti in Ancona where the surgery was carried out early on Friday by Dr Raffaele Pascarella, director of the orthopedics and traumatology division.
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Movistar Yamaha revealed there were no complications during the operation which involved fixing the fractures by inserting a metal pin, described as a locked intramedullary nail.
"Yamaha would like to thank the entire staff of the Ospedale Civile di Urbino and Ospedali Riuniti di Ancona for their dedication and professional care," the team added in a statement.
Rossi said the surgery had gone well.
"This morning, when I woke up, I felt already good. I would like to thank the staff of the Ospedali Riuniti in Ancona, and in particular Doctor Pascarella who operated on me.
"I'm very sorry for the incident. Now I want to be back on my bike as soon as possible. I will do my best to make it happen!"
The accident is not the first off-track incident to hamper Rossi's attempts to land an elusive 10th world title, with his last championship success coming in 2009.
Rossi, who also suffered a bad break to the same leg in 2010 during a practice crash at Mugello, suffered injuries to his chest and stomach in May in a motocross accident.
Rossi is fourth in the MotoGP standings with six rounds left, sitting 26 points behind leader and compatriot Andrea Dovizioso. The next race is on September 10 in Misano, Italy.
