New Sauber driver Nico Hulkenberg said eighth place in Malaysia last Sunday was "unacceptable".
After moving over the winter from Force India, the German was unable to make his debut as scheduled in Australia when a terminal fuel tank problem developed 90 minutes before the race.
But at Sepang, he finished four places ahead of rookie teammate Esteban Gutierrez, collecting four points and declaring himself "quite happy".
According to German-language sources Blick (Switzerland) and Auto Motor und Sport (Germany), however, Hulkenberg told his team after crossing the line that the result was "unacceptable".
"Thank you guys," he is quoted as having said by radio, "but we have to improve in many areas. This is not acceptable."
Blick quotes the 25-year-old as adding: "When I go at the speed of Ferrari, Lotus and McLaren, I quickly ruin the tyres. Now we have a lot of data and impressions and I hope we can draw the right conclusions."
Auto Motor und Sport quotes Hulkenberg as saying: "There are still some areas where there are gaps. The car does not fit me like a shoe."
Meanwhile, France's RMC Sport claims Romain Grosjean appeared "visibly reassured" after the chequered flag in Malaysia, having struggled with essentially the same Lotus that powered Kimi Raikkonen to victory a week earlier.
Grosjean finished sixth at Sepang, one place ahead of his Finnish teammate, but French commentator Patrick Tambay concluded: "Romain was on par with him (Raikkonen)" all weekend.
"We have two strong drivers," agreed team boss Eric Boullier.