Neither McLaren driver made it through to Q3 in Bahrain, with Lando Norris qualifying 13th and Daniel Ricciardo down in 18th as they both struggled for a lack of grip around the Sakhir track.
The team's performance has been a surprise following a hugely encouraging first pre-season test in Barcelona , where it appeared that the squad could be on a par with Ferrari.
Technical director James Key thinks a number of factors have come into play, with its car appearing not to be especially well suited to lower speed corners.
"Barcelona was pretty positive for us and I think it's a track which played towards the strengths that we currently have," he explained.
"Here, I've got to say we're obviously surprised. It's been a tough weekend for us, and we definitely came in on the backfoot, having not tested properly at all last week, because of these brake issues that has crept up on us."
Key said the brake problems, which limited McLaren's running in the Bahrain test, had huge knock-on consequences in meaning the team could not get through the programme it had planned.
"We knew we're a little bit tight on the front brake temperatures, but here, they just rocketed to a level we didn't predict. So that took a lot of learning and time to fix," he said.
"But of course, it also meant we were stuck in the garage trying to allow the car to run reliably with temperatures regenerating. So we didn't really do any optimisation for Bahrain: certainly none of what we really plan to do.
"We got some laps, and we did some reliability runs. But we didn't have the time we really wanted to both allow the drivers to get a feel for the car, at a very different track and to do a lot of work on our set-up with the new bits that we put on as well. So I think we came in on the backfoot."
But despite understanding the factors that have held the team back, Key admitted the McLaren MCL36 is lacking grip.
"There's performance to find here," he said. "I'm not going to stand here and say, 'everything's perfect', because for sure this track in particular, we've just sort of highlighted some of the weaknesses we've got. And it's a general thing.
"The car itself, the platform is working well. Mechanically, it's fine. Aerodynamically, it does what it's supposed to do. The drivers aren't finding the extremes we had in past years with different rates of balance and difficulty. So it's a lot more consistent in that respect.
"What we need is more grip, we need more grip, mechanically, we need more grip aerodynamically. And that showed itself less on Barcelona because the nature of the corners there works better with our car than the nature of the corners here."
