Sainz believes the Woking outfit has surpassed Toro Rosso in terms of speed as the two teams battle for sixth in the constructors championship.
Sainz finished last Sunday's Hungarian GP in 8th position, a result achieved for the third time in succession, just behind the slightly faster McLaren of Fernando Alonso.
"Obviously Fernando got us at the start, and then it wasn’t possible to pass him?" Sainz said.
"I think he had a couple of tenths on us, and it was impossible to get closer than three seconds. I think he was controlling us.
" McLaren has definitely got ahead of us. It’s a bit worrying knowing how much of the season is left, and their development rate.
"But we will try to compensate that and we will try to go as fast as possible and see if we can pull them back."
Toro Rosso should be implementing an update on their STR11 challenger next weekend, which Sainz is hoping will improve his performance on a track he has yet to race at.
"We have a step hopefully coming on in Germany. There we should do a step forward in performance.
"This will be the first time I drive the Hockenheim circuit in a Formula 1 car and I'm looking forward to it. I've prepared well with plenty of time in the simulator, which has given me a good idea of what to expect."