Motegi, Japan - Local rider Kohta Nozane will replace Jonas Folger for Sunday's Japanese MotoGP race at Twin Ring Motegi after the German suffered a mystery energy-sapping illness.
Folger's participation in the following two rounds is also uncertain after he was told to return home to Munich immediately for medical checks.
Tech3 Yamaha team manager Herve Poncharal said on Thursday. "He has a disease but we don’t know exactly what it is. If there is anything good about this situation, it is that we now know why he has been so weak since the summer break.
"The more he trained, the worse he felt, and the peak of the problem was on Tuesday and Wednesday where he had to stay in bed because he couldn’t even walk."
Folger, a MotoGP rookie and only German in the top flight of Grand Prix motorcycle racing, said even leaving his hotel room had been impossible.
"I’ve had Epstein-Barr virus in the past," he added. "It’s possible that this has flared up again and resurfaced, but we can’t be sure until I’ve had these tests in Munich."
Epstein-Barr causes mononucleosis, which can leave the sufferer feeling tired for weeks or months.
Folger is currently 10th in the MotoGP standings. Australia and Malaysia follow directly on from the race in Japan with Valencia ending the season on 12 November. Poncharal said it was 99 percent certain he would also miss Australia and it was too early to say when he might return.
Nozane has never raced in MotoGP, although he competed in Moto2 in 2012 as a wild card in Japan, but is a Yamaha test rider with world endurance experience and knows Motegi well.