After Ferrari reserve Robert Shwartzman ran in the car during FP1 and classified 19th, Sainz clocked 16th in FP2. Team-mate Charles Leclerc finished 16th and then 11th.
Both Scuderia drivers have called on the team to make major gains overnight, with Sainz supposing that Ferrari has again been caught out by a high-downforce track.
He suggested Ferrari's larger rear wing specification could not match the loads generated by rivals. This reminded him of the Hungary GP when he qualified 11th.
He told F1TV: "Unfortunately, we don't look very competitive yet. We're going to need to find some lap time, some downforce, some balance because today was a tough day for the team.
"I think we saw something similar in Hungary where we go to higher downforce tracks, for some reason we don't pick up as much downforce maybe as the others when we go to the bigger rear wings.
"A bit of homework to do on our side [to understand] why we slide more than the others.
"If we want to be fighting for a podium [in qualifying] and on Sunday, it needs a very big jump in performance."
Sainz warned that he could again miss out on Q3. He said: "There's a margin for improvement but when you look back at Hungary, we were fighting to get into Q3.
"It could be something similar this weekend, especially with how tight the field is."
Leclerc added: "We've had quite a few difficulties. FP2 went in the right direction but we still need to do a step forward for tomorrow.
"The feeling wasn't great, there's a big margin to improve but now we to do the step forward and find what was wrong for tomorrow."
Asked how major the changes needed to be to revive Ferrari's pace, Leclerc said: "I think it's fine-tuning because already from FP1 to FP2, we did some small changes and it went in the right direction.
"So, we need to do the same for tomorrow. Now it's all about trying to put the car in the right window, which we don't have yet.
"But I have a very clear idea of what I want from the car. Then we need to find the right set-ups to go around our issues. I'm confident. Normally we are quite good at that.
"We just need to do a step forward… We are not maximising our package at the moment."
