James Hinchcliffe will need to undergo additional medical evaluations before being cleared to practice for the Indianapolis 500 after suffering concussion in Saturday's IndyCar race on the track's road course.
The Andretti driver, who is believed to have been struck by debris, was taken to Methodist Hospital in Indianapolis where CT scans of his head and neck confirmed the injury.
He was released a short time later, but will need to be reevaluated by IndyCar's medical director Dr Michael Olinger before he is allowed back into the cockpit.
"I'm a little stiff and sore and I'd love to be back in the car tomorrow, but I suppose I should probably let the doctors make that decision," said Hinchcliffe.
Practice for the Indy 500 begins on Sunday, with qualifying scheduled for the May 17-18 weekend.
In the past IndyCar has traditionally sat drivers out for at least seven days following concussions, meaning that the Canadian's preparations for the race could be significantly curtailed.
AUTOSPORT understands that Andretti Autosport has EJ Viso on standby to fill Hinchcliffe's seat if required.
AJ Foyt driver Martin Plowman also reported being hit in the helmet by debris earlier in the race. He was unhurt, which he attributed to the new Zylon visor extensions that the series has introduced this year.
Meanwhile, Indianapolis city mayor Greg Ballard, who waved the green flag at the start of the race, received treatment after sustaining what were described as minor soft tissue injuries when he was hit by shrapnel produced in the startline crash involving Sebastian Saavedra, Carlos Munoz and Mikhail Aleshin.