Nicky Hayden's former MotoGP team-mate Valentino Rossi says it is difficult to accept what happened to him.
Hayden is in critical condition with 'serious cerebral damage' after being hit by a car while training on his bicycle in Rimini, Italy, on Wednesday.
"Very bad news yesterday," Rossi said at Le Mans, where riders have gathered for the French MotoGP.
"Sincerely it's difficult to realise, but I spoke straight away with a friend, a doctor, who said the situation from the first moment was very difficult for Nicky. Now after a bit more time you start to understand.
"It is a great shame because Nicky is a great rider for sure – a great talent, a world champion and always at the top – but especially, a very good guy. He comes from a good family of racers, his fathers and brothers, is always smiling. It's very bad.
"The situation is very difficult but we all hope for the best."
Rossi was Hayden's team-mate at Honda in 2003 when the American first joined the championship. The pair were later reunited at Ducati in 2011 and 2012.
Hayden also partnered reigning world champion Marc Marquez at Repsol Honda when he stood-in for the injured Dani Pedrosa at October's Australian Grand Prix.
"I've known Nicky since 2008, I arrived in the Repsol KTM team in 125 and he was a really nice guy to me," said Marquez.
"In Australia last year we spent a lot of time together. It was really nice, especially we did an incredible party on the Sunday! We enjoyed it a lot. I remember he was going to sleep but then he stayed until 6am!
"Like Valentino said, it's really sad. Because it's something you never expect. You take big risks every weekend on the motorbike and then something unexpected like that can happen. I wish the best for him and all his family."