Carlos Sainz has said that he is worried Toro Rosso has suddenly become the Formula 1 backmarker.

The junior Red Bull team expected a hard weekend at Spa, but Spaniard Sainz said that Toro Rosso is really "struggling" with its 2015-spec Ferrari engine.

"It will not be like Hungary and Silverstone, getting in Q3, instead we try to avoid that Manor or Sauber are ahead, as they seem to be right now," he told Marca.

"And Haas is now a good step ahead. The gap is too big."

Meanwhile, Sainz was one of several drivers who tested the Halo device on Friday, and he said afterwards: "It's obviously a strange feeling at first, but it does not impede visibility and by the end of the lap you forget you've got it.

"I think getting in and out of the car is the difficult part, but that's for the FIA to continue analysing and improving.

"The only concern I had was visibility and this was not a problem, even in Eau Rouge."

Sainz is 11th in the driver rankings ahead of the Belgian Grand Prix.