Teammate Massa, once the long-time Ferrari star, is the best part of a full second behind around the Hungaroring.
But the Brazilian insists a comparison in Budapest is unfair, after a spate of incidents including his first-corner rollover crash just a week ago in Germany.
Massa is quoted by Auto Motor und Sport as saying the crashes have meant that, on the eve of the mid-season summer break, Williams has run low on spare parts.
In Hungary, for instance, he is using a pre-Monaco grand prix floor.
"On paper it costs us two tenths," said Massa. "The rear is very unstable and as a result I am lacking confidence."
At the same time, however, Williams is happy that upgrades brought to Hungary are keeping the FW36 competitive even at the tight and twisty Hungaroring, where the ultra-quick Mercedes engine is less dominant.
Bottas played down the likelihood of a debut race win any time soon, but Williams' technical boss Pat Symonds senses potential greatness in the 24-year-old.
"In many ways Valtteri reminds me of (Fernando) Alonso," he told Germany's Auto Motor und Sport.
"You notice that he is very mature for his age, and that he has the makings of a great racer," said Symonds, who worked closely with Spaniard Alonso at Renault a decade ago.
"He (Bottas) is fast, hardly makes any mistakes, is incredibly clever.
"If he was able to win a grand prix this season, it would be a result with an eerie parallel to Fernando," he added, referring to the fact that Alonso made his victory breakthrough in his second F1 season back in 2003 -- in Hungary.