Casey Stoner will miss the next three MotoGP rounds as he recovers from the physical problems that have plagued him since the Catalunya GP in June.
The Australian will be replaced at Brno, Indianapolis and Misano by Mika Kallio, who has raced for the Pramac customer Ducati team so far this season, and is scheduled to return for the Portuguese GP in October.
"After five extremely difficult races due to my health, I returned to Australia to visit the sports doctors who have looked after me for many years," said Stoner. "We have taken the difficult decision not to contest the next three rounds of the championship, to allow my body time to recover from the recent stress.
"The doctors believe that during the Barcelona race I was suffering from a virus, and that I subsequently pushed my body too hard, leading to problems that have caused my fatigue since then. The doctors are continuing with many tests to try to understand these problems and make sure it does not happen again.
"I have spoken with Ducati and thank them for their understanding at this time. I feel very sorry for the factory, my team, my sponsors and the fans, and I am also disappointed because the bike in the last races has been very competitive. I will be doing everything possible to come back at full strength for Portugal."
Kallio is looking forward to the opportunity to prove himself on a factory Ducati in the coming races.
"When I took the call I could hardly believe it," said Kallio. "I am proud to be racing the next three events with the Ducati team.
"Obviously I'm not expecting to suddenly be a second quicker, because our bikes are very similar to the official machines, but I hope to be able to pick up some good results for the team.
"I would particularly like to thank [Pramac team boss] Paolo Campinoti, who has allowed me to enjoy this incredible opportunity. It will be a real honour for me. Nonetheless I would like to send my best wishes to Casey and I hope to see him back out on track as soon as possible."
Kallio's place at the Pramac team will be taken by Michel Fabrizio at Brno this weekend.
Ducati team boss Livio Suppo added: "It's obviously difficult for us to contemplate the next three races without Casey, but it's clear that the most important thing at this time is his health.
"We have already overcome tough times together, like when he had to undergo surgery on his scaphoid towards the end of last season. We are united whether in times of success or in times of difficulty. We are confident that the doctors will help solve the issues that have affected him since early June and that Casey will be able to take part in the last races of the year in peak fitness.
"A big thank you to Paolo Campinoti who, as always, has proved to be a great supporter and an ideal partner for Ducati, allowing Mika to ride with the Ducati team for the next three races. Mika is having a really positive debut year in MotoGP and we are delighted that he will be alongside Nicky Hayden in the official works team."