"When the drivers went to push, it wasn't there. They both said they thought it was there. They said 'We can go out and go half a second quicker'.
"They thought there was plenty in the bag but they went to push and it wasn't there and we need to understand that."
With Silverstone also hosting next weekend's 70th Anniversary Grand Prix when temperatures are again expected to climb, Green added: "There's something we do need to look at.
"It is definitely a change on climatic conditions, that is probably a big factor in why we lost some relative pace so we are going to look into that.
"But where the conditions are again different for the race and will be different again next week, so we do need to get on top of that.
"There were also question marks over power unit deployment and were we utilising that to the best of our abilities. There are lots of areas that we didn't get right and we can see that we have underperformed, for sure."
With Hülkenberg stepping in at the last minute, Green understands that even though the car is off the pace, the German's performance was expected.
"I think he has done a great job to get to where he has got to in such a short space of time, but with Lance, we were definitely expecting a little bit more," assessed Green.
"We looked at the track and the characteristics of the track and where we know the car to be strong and we really did think the car was going to be as strong as it was in Hungary.
"I think we are a few tenths of a second from where we think the car should be, and that is what we are going to be working on in the coming week."
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