Fernando Alonso is under observation in the intensive care unit of the Hospital General de Catalunya following his crash during testing at Barcelona.
The Spaniard spent the night at the hospital in Sant Cugat after suffering concussion when he crashed against the inside wall at Turn 3 on the final day of the second test of the 2015 pre-season.
Alonso was conscious and "fine", according to McLaren, but the team has not yet fully explained the circumstances of the accident.
The two-time world champion has been in the intensive care unit, as part of the protocol for patients with concussion.
His manager Luis Garcia Abad said the Spaniard could stay under observation for 48 hours.
Alonso's presence in the final test of the pre-season is very much in doubt, the running kicking off on Thursday.
McLaren has Kevin Magnussen, its race driver from 2014, and GP2 star Stoffel Vandoorne as test drivers should it require anyone beyond Alonso's teammate Jenson Button.