Valentino Rossi has returned another positive coronavirus test ahead of the European Grand Prix this weekend.
The Italian said he only "felt bad" for two days after his initial diagnosis and has been "fully fit" since, but recorded another positive result on Tuesday.
He must test negative in PCR tests on Wednesday and Thursday if he is to be cleared for a MotoGP™ return in Valencia.
If he does not participate, Monster Energy Yamaha have confirmed American Garrett Gerloff – who rides for the manufacturer in the Superbike World Championship - is their standby replacement rider.
Maverick Vinales was the team's sole representative for the Aragon and Teruel Grands Prix, which Rossi missed.
The development has compounded a miserable second half to the season for Rossi.
Prior to catching the virus and missing races, the 41-year-old had three consecutive retirements, having previously recorded four top-five finishes to start the season strongly.
"This virus is very complicated and serious," Rossi said. "I felt bad for two days, then in a few days I came back to being fully fit, at my 100 per cent.
"I self-isolated at home all the time and I followed the medical advice closely. It is a very sad and difficult situation, but that is the way it is.
"Unfortunately, [on Tuesday] I had another test and it came back positive again, like all previous ones. Luckily I still have two more chances to be back on track on Friday or Saturday.
"I am very sad because I am feeling well, and I can't wait to be back aboard my M1 and be reunited with my team.
"I really hope the next PCR test result will be negative because missing two races was already two too many."
Seven-time world champion Rossi will leave the factory team next year to join Petronas Yamaha, swapping seats with Frenchman Fabio Quartararo.