Tech 3 Yamaha rider Jonas Folger will miss this weekend's Japanese MotoGP round at Motegi through illness.
The French team announced on Thursday that Folger will have to fly back to his native Germany to undergo medical checks for a presumed mononucleosis in Munich.
His participation in the subsequent Phillip Island and Sepang rounds has also been described as "doubtful", with team boss Herve Poncharal saying it was "99%" certain he will not race in Australia.
Yamaha test rider Kohta Nozane has been drafted in as a replacement for Folger at Motegi and will thus make his MotoGP debut this weekend alongside Johann Zarco.
"I've been feeling really weak since the Misano and Aragon rounds," admitted Folger.
"On arrival here, I have been struggling with my energy levels, even leaving the hotel room was impossible for me.
"I met with Herve on Wednesday night and we agreed to go for a check with the doctors this morning.
"I met with Dr Charte and the Clinica Mobile team and they have advised me to return to Germany right away for a series of blood tests.
"I've had Epstein Barr virus in the past and it's a possibility that this has flared up again and resurfaced, but we can't be sure until I've had these tests in Munich."
Folger currently lies 10th in the standings with a best finish of second at his home race at the Sachsenring, and is 33 points behind team-mate Zarco.
His lay-off all but guarantees Zarco this year's 'Rookie of the Year' title.