Haas has replaced Esteban Ocon with reserve driver Ryo Hirakawa for the rest of the Mexico City Grand Prix weekend.
The surprise switch, announced on Saturday morning in Mexico, gives the 32-year-old Japanese driver his Formula 1 race debut in Sunday's 71-lap race.
No reason given
Haas announced the change in a short statement that ended: "No further statements will be made at this time."
The team has given no reason for the decision. It has confirmed only that Ocon is not ill.
Ocon drove in just one session in Mexico. Hirakawa took his car in first practice on Friday, as teams must give practice running to rookies, and Ocon was 13th in the second session.
That session included a radio dispute between Ocon and his race engineer, Laura Mueller, over whether he should run the soft or the medium tyres.
Who is Ryo Hirakawa?
Hirakawa is a Le Mans 24 Hours winner and a two-time World Endurance Championship title winner with Toyota.
He has been Haas's reserve driver, and his run on Friday was his third practice outing of 2026, in which he finished 15th.
He now partners Oliver Bearman for the rest of the weekend. In qualifying on Saturday he was knocked out in the first session in 19th place, four places behind Bearman.
What it means for Ocon
Ocon was already on his way out of Haas. The team has chosen Ferrari junior driver Rafael Camara to replace him in 2027, ahead of Hirakawa.
Haas has not said who will race in its second car at the next race, in Brazil.
The Mexico City Grand Prix starts at 1:30am on Monday 2 November, India time.
Race it on Late Brake
Mexico City is not one of Late Brake's circuits, but the next race is. Interlagos is one of six real circuits in the free browser game, where you can take on AI rivals or race other players online before the season heads to Brazil.
Sources: BBC Sport, Autosport, The Race, The Race qualifying results