Seven-times world champion Michael Schumacher, who retired in 2006, still has to be Ferrari's first choice to stand in for injured Brazilian Felipe Massa, former champion Niki Lauda said.
"Who is available? There is nobody available anywhere near Michael's performance. I was thinking about it, and there is nobody," the retired triple champion told reporters before Sunday's Hungarian Grand Prix.
Schumacher, 40, Formula One's most successful driver with 91 wins, hung up his helmet at the end of 2006 and the German has shown no signs of wanting to come back despite the occasional bike race.
His manager Willi Weber told Germany's SID news agency that he doubted the champion would consider coming back.
"I don't see it," he said. "Ferrari have two good test drivers who are keen to sit in the car if it is available."
Ferrari are almost certain to need a replacement for Massa after the 28-year-old suffered a serious head injury after being struck on the head by debris in Saturday's Hungarian Grand Prix qualifying.
Massa was in a stable condition after emergency surgery but the hospital doctor attending him said on Sunday he was expected to remain under sedation in intensive care for the next 48 hours.
Formula One takes a three week break after Budapest before the next race in Valencia, Spain, on August 23 and then Belgium on August 30.
Lauda, who came back remarkably quickly in 1976 from a near-fatal crash, said it could take two to three months before Massa could think about racing again.
"He needs to take a decision, and for the whole business of Formula One, which is ill anyway because nobody is coming, he [Schumacher] would improve it by 20 to 30 percent interest," added the Austrian.
"But this is a question nobody can answer because nobody knows what Schumacher is doing."
Lauda, who won two of his titles with Ferrari, said that if he was Ferrari president Luca di Montezemolo then Schumacher would be the first person he would ring.
"Who else is he going to ring? The test drivers are useless, and there is nobody else here if you think about it," said the 60-year-old.
"But this is a principal decision of him [Schumacher], for him to consider this. If he doesn't want to, the idea is finished in the same second," said Lauda.
Ferrari has 38-year-old Italian Luca Badoer, who started 49 races for Scuderia Italia, Minardi and Forti without scoring a point between 1993 and 1999, and 35-year-old Spaniard Marc Gene as their official reserves.