Lewis Hamilton and Jenson Button headed into the Silverstone weekend expecting the high-speed characteristics of the track to perfectly suit the MP4-27.
But their form was hit by a lack of grip in both qualifying and the race that left them at the bottom end of the top 10, while Red Bull and Ferrari dominated the fight at the front of the field.
Reflecting on McLaren's struggles after the race, Whitmarsh said that the pressure was now on for the team to rediscover its winning ways, and that developments planned for Hockenheim must be a good improvement.
"I think it's been one of those weekends," said Whitmarsh. "We had lots of stuff on the car, and we couldn't run it properly on Friday.
"We've got a decent, bigger, more visible upgrade package for Germany and we've got to deliver that and make that stick. It's the same old game, we have got to develop the car, and we have got to make sure we use the tyres better."
The upgrades scheduled for Germany are understood to include a new bodywork concept at the rear of the car, as well as further refinement to the adjustable front brake ducts.
Such progress is going to be vital, with Jenson Button suggesting that the outfit had fallen behind Williams and Sauber in terms of overall pace at Silverstone.
Whitmarsh added: "It is always disappointing if you are not competitive and certainly at the end of the race we were not, and through no fault of the drivers. Both of them did a good job.
"We weren't quick enough, period. We didn't get as much data this weekend as we wanted.
"We have got to get that data; we got to make sure we come with the planned decent update for Germany. We've got to work hard to update the car, we have got to understand the tyres and we have got to minimise mistakes.
"The average of all three pit stops was sub three seconds, which is something we have been working on, as I think you know. But fundamentally, good pit stops, a good couple of drivers, solid drives, bad qualifying, but no events upon which we could capitalise. Just not quick enough is the school report."