Jenson Button fears he hasn't spent enough time in his MP4-28 especially as rain is expected at Barcelona on Friday.
The McLaren driver replaced his team-mate Sergio Perez for the final two days of running at the Spanish track.
However, Button's track-time on Thursday was curtailed due to set-up and mechanical changes. He completed just 71 laps.
The Brit will be in action again on Friday but with rain forecast Button fears McLaren may be falling behind.
"It's going to be wet tomorrow so it's tricky," he said.
"I don't feel like I've had enough laps in the car and I think it's the same for Checo [Perez] as well.
"We're doing a lot of work to really understand the car in terms of the aerodynamics.
"We were doing a lot of rake work in the morning and it always takes a little bit longer than you expect. You lose a lot of time doing that.
"It's important for us if it helps us throughout the year to understand this car and its direction but it does limit your testing."
Asked whether he felt McLaren were behind in their preparations, he said: "In terms of laps completed and the testing that I have achieved, yes, I think we're behind on that.
"It's not been perfect and we know we've got a lot of work to do in terms of spending more time out on the circuit and understanding the car."