Felipe Massa says that Williams needs to get to the bottom of its tyre problems after both he and teammate Valtteri Bottas struggled for pace on the soft Pirellis at Spa.
The Grove squad's cars found speed after switching to mediums, and Massa was able to recover to sixth place, although Bottas was hampered by a drive-through penalty for having his tyres fitted incorrectly.
“I'm not happy because I would say maybe the first 10 laps was a disaster for us,” said Massa.
“The start was not good, but also the pace on the soft tyres was really a disaster; we lost many positions with both cars. It was really tough.
"I'm sure it paid a lot the result we had at the end of the race, this problem that we had at the beginning. I'm not happy because the race could have been a completely different result at the end.”
Massa said he didn't see a trend, despite the team also struggling at the previous race in Budapest.
“I think definitely Hungary was not a track for us," added the Brazilian. "Before that we were doing a great job.
"It was an issue for those tyres on this track, and we we need to understand why it was not working, and make it work [with] whatever tyres we use from now to the end of the season.”
Regarding the next race at Monza, he said: “We always have big hopes at Monza - a fantastic track, and there's a lot to concentrate on so we can try to do the best we can for that track.
"Last year I finished on the podium, I hope we can see something similar this year.”