Haas will not be able to call upon Jack Doohan for the remainder of the Formula 1 season, despite the Australian serving as one of the American outfit's reserve drivers.
The limitation stems from the sporting regulations, which permit a team to use a maximum of four drivers in races throughout a single season. Haas have already fielded three – Oliver Bearman, Esteban Ocon, and Ryo Hirakawa – while Rafael Camara is set to make his Formula 1 debut in São Paulo this weekend.
Doohan's only avenue to race for Haas this season would be via special dispensation from the FIA. Such permission would only be granted in exceptional circumstances, such as a race weekend injury to one of their drivers where Haas has no viable alternative substitute available.