The team said in a statement on Wednesday they had brought in Italian Mattia Pasini as a stand-in for the second-tier race at Misano because Martin "is still to test negative".
"I'm very sorry about what Jorge Martin is going through. I hope he can come back soon and in the meantime I will look after his bike well," said the 35-year-old Rimini-born rider who raced in MotoGP in 2012.
Martin has had to self-isolate after becoming the first rider in MotoGP and its support races to test positive for the virus.
Spanish media said he was asymptomatic and was continuing training at home in Andorra.
He missed last weekend's San Marino round at the same circuit and has now dropped from being eight points behind championship leader Luca Marini to fourth overall and 33 points adrift.
Racing Point's Mexican Formula One driver Sergio Perez also missed two grands prix last month after testing positive for COVID-19. (Reporting by Alan Baldwin, editing by Toby Davis)