Eddie Irvine says Jenson Button will forever be considered second-rate unless he convincingly beats new team-mate Rubens Barrichello in 2006.
Button has not proved himself a top-flight driver, says Eddie Irvine
Former F1 driver Irvine says Button has too often been outpaced by team-mates.
"Jenson will either confirm his talent or confirm his lack of pace this year," Irvine told Motorsport News.
"Rubens is the perfect measure for him. Can Jenson step up to the mark or be relegated to being a number two for the rest of his career?"
Barrichello has spent the last six years as team-mate at Ferrari to Michael Schumacher, a position he took over from Irvine who had previously partnered the Brazilian at Jordan.
Irvine believes Button's unconvincing performances when team-mate to Italians Giancarlo Fisichella and Jarno Trulli at Benetton and Renault in 2001 and 2002 have left big question marks about the Englishman's speed.
"Never has a world champion been blown off by all his team-mates," Irvine said.
"Jenson has been beaten by all of them apart from Takuma Sato and Jacques Villeneuve. I have to keep looking at that - it is evidence you can't ignore.
"I'd employ Jenson as a number two driver. His strengths are he doesn't make mistakes, rarely goes off the track and brings it home all the time.
"I just worry about his pace because Trulli killed him and Fisichella killed him and there was nothing between him and Jacques really and Jacques was totally unfavoured in that team (BAR)."
Irvine also said in the interview that he expected the 2006 season to be "the best in a long-time", with a three-way fight for the title between veteran Michael Schumacher of Ferrari, new champion Fernando Alonso at Renault and McLaren's Kimi Raikkonen.
He said Alonso and Raikkonen were "right there in the (Ayrton) Senna league".
He added: "The three best guys are going to be in the three best cars. Michael is up against guys who are half his age and have twice the balls."
But Irvine said Schumacher, a seven-time champion, still had the edge on his younger rivals.
"Michael is still a 9.1 out of 10 and the other guys are a nine. But age doesn't matter. You don't lose it. I was better in my last year than I was at any stage.
"Michael isn't getting any slower. If he'd had a better car, he would have won 11 titles by now."