Mexican rookie Sergio Perez was forced to pull out of this weekend's Canadian Grand Prix after feeling unwell following Friday morning's opening free practice session.
The 21-year-old driver from Guadalajara, who crashed heavily at the Monaco Grand Prix two weeks ago, was only passed fit to take part on Thursday.
But after completing 27 laps on Friday morning he told the Sauber team he did not feel 100 per cent fit and did not want to continue. He had suffered from severe concussion following his accident in Monte Carlo.
The team approached McLaren to ask if Spanish veteran reserve driver Pedro de la Rosa could substitute for Perez -- and he took over in time for Friday afternoon's second session.
Perez said: "Of course I wanted to drive and I had no doubt I was alright, but, apparently this is unfortunately not the case. I am deeply disappointed.
"I spoke to the team after the session and told them that I'm not feeling 100 per cent fit. I only want to drive when I'm 100 per cent well. I need some more time to recover."
De la Rosa drove for the Sauber team last season before being replaced by German Nick Heidfeld, now with Renault, for the final five races of the year.
Team chief Peter Sauber said: "This development hit us by surprise because Sergio underwent thorough medical checks, first in the hospital in Monaco, then in a hospital in Zurich, and finally the FIA doctors gave him the green light this Thursday in Montreal.
"Nobody could have foreseen that he would feel unwell. Maybe we are being overly cautious, but when it's about the health of our drivers we take zero risks.
"We want to thank Pedro de la Rosa and McLaren for helping us out in this situation."
De la Rosa will wear his McLaren team overalls while driving for Sauber this weekend.