Stoffel Vandoorne has declared the MCL32 is "impossible" to race and "urgently requires" more power.
The double DNF that many were expecting from McLaren came true when Vandoorne and Alonso were both forced to retire at the Chinese Grand Prix, with the latter once again running in the points.
Vandoorne, though, bemoaned the lack of pace on the straights and has pleaded for a quick response.
"The car feels pretty good. When I was behind the Williams of Massa, I even commented that I was faster in every corner," Vandoorne told Belgian newspaper Het Laatste Nieuws.
"Our top speed on the straights is too low for racing. It's impossible for us. At the start of the straight you look in the mirror and by the end a car is next to you. We urgently require more power Even with DRS open we can't pass anybody."
"The engineers said our potential laptimes were similar, but the difference was the groups we were in. Up front, the pace is higher but further back, where I was, you lose time because there are more slow cars."
Back-to-back weekends of racing is less than ideal for McLaren, and Vandoorne is expecting the beleaguered Woking team to run into more problems in Bahrain.
He added: "It's the second race of the year, we know it's not easy with our car now but we try the best we can and there will be improvements soon. But it will probably not be for Bahrain — I think it's going to be a bit of a difficult weekend for us again."