The Brit, who started last weekend's Chinese Grand Prix at the top of the grid and eventually finished third in the race, was 10th fastest in the second practice in Bahrain on Friday.
"It's not looking like that at the moment," BBC Sport quotes him as saying. "We had some problems in P1 with the car so I was really struggling with the balance.
"We made some changes and it improved for the second session but in terms of pace we were struggling, the balance is not really where I want it to be so we have a lot of work to do tonight."
Lotus's Kimi Raikkonen was the quickest in the second practice.