Button could only finish ninth in Sunday's Spanish Grand Prix, a place and seven seconds behind team-mate Lewis Hamilton, who had started 14 positions back at the rear of the grid.
Hamilton was punished for a fuel irregularity in qualifying after the 27-year-old had thought he was again starting on pole at the Circuit de Catalunya, only for his times to be discounted.
It was the latest mistake from McLaren to undermine one of their drivers during a grand prix weekend as a considerable amount of points have been lost from strong positions.
In Button's case in Barcelona, however, he simply could not find any speed in his car, and that is now a major worry.
Speaking to Press Association Sport, Button said: "It was a tough weekend, especially on Sunday.
"The most important thing is to understand why I didn't have any pace over the weekend on low and high fuel.
"Even if qualifying doesn't go well, normally we can get some good points in the race, but I couldn't look after my tyres and I didn't have any pace. It's something that's a concern.
"Yes, Lewis did a fantastic job in qualifying, and the team did well to produce a car like that.
"In the race you can say Lewis again did a good job by finishing eighth, which he should be happy with, but still the pace isn't there.
"It was a worse weekend for me, but if you look at Lewis' pace in the race, I still don't think it's where you would expect us to be.
"My fastest lap of the race was 2.6secs slower than the quickest overall, which is just strange, and I don't understand why that is.
"And it wasn't one end of the car I struggled with, it was both. I've never been in a position like that. It was very extreme."
With Williams' Pastor Maldonado claiming victory, for only the second time in Formula One history five different drivers and five different constructors have won the first five races.
Button is scratching his head at that one, adding: "It's not just us.
"The Red Bulls did a better job at the weekend than us in terms of points, but still they weren't quick when you compare them to Williams, Sauber, Lotus and Ferrari.
"Five different teams winning five different races, we really don't know what's going on, and I think that's the same up and down the pit lane."