The Woking-based team enjoyed a strong first test in Barcelona, but it all went wrong in the second week in Bahrain when it encountered brake overheating problems.
That limited its running and put it on the back foot for last weekend's F1 opener, where it also struggled with a lack of grip and neither Lando Norris nor Daniel Ricciardo finished in the points.
The team admits that the Bahrain brake issues caught it completely by surprise, and they have still not been fully resolved as overcoming the problem is not straightforward.
Reflecting on the circumstances that triggered the woes, Key says that the original brake duct design passed all the criteria that McLaren had for brakes – but other influences of the 2022 rule changes were not anticipated.
"I think, in a way, it demonstrates how fresh these cars are and how much there is still to learn about how to get them right," he explained.
"I think the brake duct thing was a really strange anomaly in that it passed every criteria we had for cooling, yet didn't work.
"You think: 'well what the hell's going on there?' So you research it and you realise there's a certain condition, which was shown by this circuit, where it just sort of switched off basically internally."
Key said the phenomenon prompted a massive reaction back at McLaren's factory to try to come up with solutions, with interim changes in place for the Bahrain GP weekend.
"The guys did a fantastic job in recognising the problem very quickly," he explained. "Coming up with solutions from an aerodynamic perspective, in terms of a duct flow perspective, getting it into design and then a massive push in production to get it here.
"It's been a while since we've had to do something like that, so it was an excellent reaction from the team.
"But it was also a bit of a lesson learned. I think it shows that not all methodologies that we've trusted in the past necessarily carry across to something so different here."
McLaren said it was currently evaluating what changes were being made to the brakes for this weekend's race in Saudi Arabia and the following event in Australia.
Team boss Andreas Seidl said: "The team is working obviously hard to bring new parts to the track now. But we're still working on exactly what comes to the track next week for Jeddah and then the race afterwards in Melbourne."
