Felipe Nasr is expecting Sauber to suffer at the next few races.
Early in 2015, the Brazilian rookie emerged as a potential star of the future, finishing fifth in Australia and scoring more points in China.
But Nasr, 22, warned before the Spanish grand prix - where he qualified 15th - that a lack of performance upgrades could mean Sauber begins to struggle.
"I think I’m doing my best with what I have," he said after the Barcelona round.
"I think I capitalised well in the early races to score as much as possible when I had the opportunity."
Sauber, with probably the second-smallest budget in F1 and having struggled for survival over the winter, benefitted early in 2015 from Ferrari’s vastly improved engine.
But the Swiss team then arrived in Spain where key rivals unveiled substantial performance upgrades.
Nasr, meanwhile, said: "I’m driving the same car with which I started the year in Australia. There’s nothing different, just small details."
He said the C34 car will only be upgraded "mid-season". UOL Esporte said the upgraded Sauber is expected no earlier than Belgium in late August.