Rider Alex Marquez suffered a fractured neck and a broken collarbone after a high-speed crash at the Catalonia MotoGP.
The race was twice red-flagged after incidents before eventually being won by Fabio di Giannantonio in Barcelona.
The Italian Ducati rider was able to take the flag at the third attempt from Joan Mir and Fermin Aldeguer.
Two big crashes stopped the race, with Marquez involved in a nasty accident when he ran into the back of Pedro Acosta.
Acosta was leading the race on lap 12 of 24 when his bike suddenly lost power on the straight and the Spaniard desperately tried to warn the riders behind him.
However, second-placed Marquez had no time to react, hitting the back of the KTM before he and his Ducati bike went towards the wall and then cartwheeled through the air.
Marquez was taken away in an ambulance and was not involved in the restart, with his team Gresini Racing saying he was conscious.
The team later, external confirmed, external that Marquez has a “marginal fracture of his C7” – the seventh and lowest vertebra in the neck and will require further medical evaluation .
He also has a “right clavicle fracture” – collarbone fracture – which will be stabilised with a plate and the rider will undergo surgery on Sunday.
“Everything under control,” said a social media post from Marquez., external “Time to go into the operating room tonight, but I couldn’t be in better hands.
“Huge thanks to everyone for your concern and for the loving messages I’m receiving.”
Di Giannatonio was hit by debris in the crash but was able to restart – and win.
Former Moto2 champion Johann Zarco was also taken to hospital after crashing out of the restart, which triggered a second red flag.
Zarco became tangled up in his bike and his LCR Honda team later said, external that he had “sustained injuries to the anterior and posterior cruciate ligament, and to the medial meniscus” while he “also suffered a small fibula tear in the ankle area”.
They added: “Zarco will remain overnight under observation and will travel to France on Monday to consult with specialists and assess the next steps in his recovery.”
“Today has not been an easy day for everybody. I really hope that Alex is fine,” Di Giannantonio said.
“We’ve been really lucky. We know that our sport is amazing. We try to give an amazing show, but also we are humans and we are in danger. So I really hope that everybody is safe.”
Reigning champion Marc Marquez – Alex’s brother – missed the race after fracturing his foot in France last weekend.