The Woking squad led the constructors' championship after a double podium in the season-opening Australian GP, but could only manage a best finish of sixth with Jenson Button in round two at Sepang.
McLaren racing director Eric Boullier expressed concern that McLaren will continue to be off the pace over the coming races, unless it can dramatically improve the MP4-29 relative to its rivals.
"I don't think it's going to be a one-off, that's the problem," Boullier said.
"We said in Melbourne we were happy with the result because we had been opportunistic.
"But obviously we knew the others were going to massively catch up once they started to fine-tune their balance and ride and everything on their cars, especially the electronic parts. It's exactly what happened."
Boullier confirmed Malaysia's hot and humid conditions had contributed to McLaren's struggles, and he expressed surprise that so few cars broke down.
"This is the down spiral when it's very hot," he added.
"Our lack of downforce is making the car slide more and when the car slides more the tyres get hotter, and if you pass the limit of the surface temperature you get in trouble.
"This is exactly what happened to us. It was very hot and we were struggling with the grip.
"Strangely there was not much reliability issue here [in Malaysia], and it's true our strategy to be reliable was good in Melbourne and here it didn't pay off because most of the cars saw the chequered flag.
"We know performance-wise we have to massively catch up and aggressively develop the car."
Button was more upbeat than Boullier post-race, stating that he believes the Sepang circuit masked the progress that McLaren has made with its car.
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- Eric Boullier
- Jenson Button