Alexander Wurz will replace injured McLaren driver Juan Pablo Montoya in the San Marino Grand Prix this weekend.
Montoya missed the Bahrain Grand Prix earlier this month
Montoya suffered a hairline fracture in one of his shoulders while playing tennis last month and scans this week revealed that he is still not yet fit.
The 29-year-old Colombian is now expected to return at the Spanish Grand Prix on 6 May.
Austrian Wurz, who is McLaren's official reserve driver, last competed in a Grand Prix in Malaysia in 2000.
Spaniard Pedro de la Rosa replaced Montoya at the Bahrain Grand Prix earlier this month - finishing an impressive fifth - because of difficulties fitting the tall Wurz into the new MP4-20 car.
But Wurz is now said to be "fully comfortable" in the car so De la Rosa will revert to third driver.
McLaren chief executive Martin Whitmarsh said: "It is disappointing that Juan Pablo is not yet fully recovered from the injury he sustained to his shoulder.
"However it is imperative that his shoulder heals properly to ensure no additional risk.
"We will assess the situation again next week but at this stage we are optimistic he will return for the Spanish Grand Prix as the shoulder is healing well."