Renault should have given Frenchman Franck Montagny a chance to race in Formula One rather than drafting in Canadian Jacques Villeneuve, according to former champion Alain Prost.
"One can regret that it was not him (Montagny) for the last three races," the four times champion told France's TF1 television after Villeneuve finished 10th in Japan at the weekend.
"Honestly, I cannot understand Renault's choice and (team boss) Flavio Briatore's decision.
"It's his driver (Montagny) who is under contract and independently of the fact that he's French, he is a reserve driver who deserved the experience."
The Canadian, who left BAR last season and had not raced for almost a year before driving for Renault in Shanghai last month, failed to score any points in China and struggled at Suzuka.
"The training (I did) just wasn't enough for this track," said Villeneuve.
Renault are 16 points adrift of second-placed BAR going in to the season-ending race in Brazil, their hopes of completing the year as runner-up effectively over.
"Claiming second place is now a mathematical possibility rather than a probability," Renault's engineering director Pat Symonds said after the weekend's race at Suzuka.
"But that does not diminish our efforts in any way...both race drivers will be testing this week at Jerez and we will run new parts in Brazil that the conditions prevented us from evaluating in Japan.
"Our approach in Brazil will be very aggressive."