Cech had been hailed as the signing of the summer, an incredible coup given that he was allowed to leave Chelsea and join their title rivals. The missing piece of a league-winning jigsaw, even.
It may eventually prove to be the case and he is certainly one of the world's outstanding goalkeepers. But not on this evidence. He did not earn points. He lost them. And Arsenal need more than Cech.
Arsenal wanted to make a statement, but this was not the one they hoped to release. Maddeningly for their fans this was one of those performances they produce now and again where they appeared to simply think they could turn up and win, perhaps fooled by West Ham's Europa League exit and recent serial failure against Arsenal.
There was a lack of drive, impetus and, dare it be said, leadership from the home side. A lack of urgency - until the blind panic of a cavalry-charge after falling two goals behind with manager Arsene Wenger throwing on Theo Walcott and Alexis Sanchez.
Wenger looked crushed, and there were the inevitable boos, although just a smattering. The Arsenal manager talked of how there was collective responsibility in the West Ham goals. Poor organisation - and terrible tracking from Nacho Monreal - was a vital part of the first and slow reactions were crucial to the second.
But Wenger knows the focus will be firmly on his new goalkeeper. Cech will bounce back. Arsenal will too. But Wenger ruefully remarked that away to Crystal Palace and home to Liverpool - their next two fixtures - did not represent a soft landing into the season.
Arsenal must be markedly better. Markedly better in defence - and this was their most experienced, with a back five all aged over 30 - and in midfield where they lacked that bit of grit and the over-reliance on Francis Coquelin was exposed.
Santi Cazorla played the first half out wide and Mesut Ozil's contribution was fitful. Cheikhou Kouyate attacked space left by the Arsenal defence and headed past a flapping Cech for the first goal. The goalkeeper looked ponderous for the second too, by Mauro Zarate.
Cech slowly trudged across the turf at the end. This was not the debut he dreamed of. It was not what Arsenal expected either.
Elsewhere, Newcastle United and Southampton drew 2-2 while a fine strike by Coutinho gave Liverpool a 1-0 win at Stoke City.