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Onana - previously a target for Chelsea - was handed a 12-month ban in February having failed an out-of-competition test for banned substance furosemide the previous October.
The 25-year-old has insisted it was accidental, having mistook his wife’s prescription medication for aspirin due to similiar packaging.
Onana has been unable to play or train with Ajax, who were crowned Eredivisie champions on May 2, since the ban was imposed - but will have his case heard at CAS on Wednesday.
Before signing Edouard Mendy last year, Chelsea were quoted a fee of £30million to land Onana. Any sale during this window would be greatly reduced given the ban and his lack of game time.
Reports this week claim Mikel Arteta could wrap up a signing for as little as €2m (£1.73m).