Pedro de la Rosa lost his race seat because he was not consistent enough.
In Singapore on Thursday, photographers clamoured around the Spaniard's replacement Nick Heidfeld, who is well known to the Swiss team after two separate tenures in the past.
"Nick will help us assess how good our car really is," team boss Peter Sauber is quoted on Thursday by Auto Motor und Sport.
"When we looked at the first 14 races, Kobayashi and de la Rosa had approximately the same speed, but not consistently enough.
"With (Kamui) Kobayashi you expect that from a newcomer," said Sauber, who has retained the Japanese rookie for 2011.
"With Heidfeld, we know that he can drive consistently. But I don't expect marvellous things from him first time out, because he has to get used to the car and tyres.
"He will show us what he can do at Suzuka," added Sauber.
33-year-old German Heidfeld, however, said he is not approaching Singapore – his first Grand Prix since Abu Dhabi last year – as a gentle re-acclimatisation to F1.
He is yet to secure a contract for 2011.
"For me this is my chance to show that I deserve to be here," said the veteran of approaching 170 Grands Prix.
Heidfeld said his target is to outclass his new teammate.
"F1 is not kindergarten," he said. "You always have to beat your teammate and you have to believe that you can.
"Who is too slow is gone, and that's how the sport is."