MotoGP medical director Angel Charte says he is "perplexed" by the speed of Valentino Rossi's recovery following his injury.
On Thursday, Yamaha rider Rossi was cleared to ride in this weekend's Aragon Grand Prix, just 21 days after he fractured his tibia and fibula in a motocross training crash.
Doctors had originally expected the 38-year-old to be out of action for 30-40 days.
The Italian had returned to action earlier this week when he tested a road bike at the Misano circuit before attempting to race this weekend.
Charte admitted Rossi's recovery had been exceptional.
"It's obvious that Rossi feels in a position to race, just like it's obvious that we need to check that he can and that he has to pass all the tests like any other rider," Charte said.
"Rossi's fracture happened while training and I must say that his doctor did an exceptional job: 24 hours later the rider was already in rehabilitation.
"We have been monitoring his recovery almost daily, but it's obvious that 'Il Dottore' is 'Il Dottore' and I have to admit I was perplexed by how well he has recovered from this type of injury."
Charte concedes Rossi, still walking with crutches, will not be 100 percent this weekend, but insists the Yamaha rider has passed the required tests without problems and is aware of the risks of having a crash this weekend.
"We'll monitor his evolution closely. He has undergone the standards tests and he has passed them perfectly," Charte added.
"He's not 100 percent, that's obvious, but he has no pain, which is very important. But we'll probably have to apply an anti-inflammatory treatment during these days.
"I spoke with Valentino before the tests and he is a man with a very clear mind and aware of the risks. He knows that a crash would not be good but I want to make clear that he been cleared to race because he's ready to race. He has passed all the tests that all riders have to pass.
"But we have been excited and impressed of how well his evolution has been so far."
Yamaha World Superbike rider Michael van der Mark had been named as Rossi's replacement should the Italian not race this weekend, and the Dutch rider will be on stand-by at Aragon.