Jenson Button said he was mystified by his BAR-Honda team's poor performance on their return from a two-race ban.
EUROPEAN GP RESULT
1 F Alonso (Renault)
2 N Heidfeld (Williams)
3 R Barrichello (Ferrari)
4 D Coulthard (Red Bull)
5 M Schumacher (Ferrari)
6 G Fisichella (Renault)
7 JP Montoya (McLaren)
8 J Trulli (Toyota)
The Englishman finished 10th in the European Grand Prix having predicted in the run-up to the race that he would be challenging for victory.
"We thought we would be reasonably strong here so the race has taken us by surprise," Button said.
"I don't understand what was going on really. I struggled a lot, and we have a lot of work to do in testing."
Button said his car lacked rear-end grip through the corners and added that he was also having problems getting it to change direction on the way into bends.
"We had a lot of oversteer in the car but also in turn-in we had huge amounts of understeer," he said.
"We have a few new aero parts coming for the next race so that will help us there I'm sure.
"Hopefully we will be able to make a bit more of an impact on the grid in Montreal."
BAR sporting director Gil de Ferran said: "This has been a disappointing day for us as our speed in testing clearly has not translated into good race pace.
"Basically, our car just wasn't fast enough. At this stage we do not fully understand why, but we will investigate this during tests at Monza and Silverstone this week.
"We clearly need to find a solution before we leave for the races in North America."