Reigning world champion Jorge Lorenzo will miss Saturday's Netherlands Grand Prix at Assen after breaking his collarbone in a high-speed crash.
The 26-year-old Spanish rider lost control in the wet conditions during Thursday's second free practice.
Lorenzo was taken away in an ambulance and it was later confirmed he would fly to Barcelona for surgery.
The Yahama rider trails World Championship leader Dani Pedrosa by seven points in the 2013 standings.
The official MotoGP Twitter feed said he would be kept in hospital for observation before travelling to Barcelona on Friday for surgery.
"Jorge has suffered an accident at over 200 kilometres an hour and the obvious injury at this point is a broken left clavicle," said Dr Xavier Mir.
"However, as this was a significant accident, we have to take into account that there could also be head, thoracic or abdominal injuries.
"For now, we have taken him to hospital in Assen where CT scans will be carried out in those three areas to rule out any further injuries.
"It would also be a prudent move to observe him over 12 hours to make sure he can be treated by the appropriate parties when he arrives in Barcelona on Friday."
Yamaha Motor Racing managing director Lin Jarvis later said the early indications were that Lorenzo could be back for the Sachsenring GP in two weeks' time.
The last time Lorenzo missed a race was when he relinquished the MotoGP title to Casey Stoner in Phillip Island two years ago.
In November 2009, Lorenzo had an operation for the removal of a titanium plate, fitted following a crash in the 250cc race at the Circuit de Catalunya in 2005, from the same collarbone.
Lorenzo set the fastest time in the dry session on Thursday morning before the rain came down.
He recorded his third win of the season with victory at the Catalunya Grand Prix on 16 June, but suffered bad luck in Assen last season when he was taken out at the first corner by Alvaro Bautista.
Lorenzo is the latest MotoGP rider to break his collarbone. Pedrosa's title challenges in 2010 and 2011 were both hampered after he missed races for that reason, while Cal Crutchlow missed his home grand prix at Silverstone in 2011 after sustaining the same injury in qualifying.
It was also announced on Wednesday that Assen - the only circuit to hold a grand prix every year since the start of the world championship in 1949 - will remain on the calendar until at least 2021.
Combined Assen practice times:
1. Jorge Lorenzo (Spa) Yamaha 1'35.263
2. Cal Crutchlow (GB) Tech 3 Yamaha 1'35.613
3. Marc Marquez (Spa) Repsol Honda 1'35.883
4. Valentino Rossi (Ita) Yamaha 1'35.958
5. Dani Pedrosa (Spa) Repsol Honda 1'36.034
6. Aleix Espargaro (Spa) Aspar ART 1'36.430
7. Stefan Bradl (Ger) LCR Honda 1'36.559
8. Alvaro Bautista (Spa) Gresini Honda 1'36.763
9. Michele Pirro (Ita) Ducati 1'36.910
10. Bradley Smith (GB) Tech 3 Yamaha 1'37.002