Two mistakes by the new goalkeeper Petr Cech sent Arsenal to a 2-0 defeat at home to West Ham, while Liverpool began the Premier League season with a late 1-0 win at Stoke on Sunday.
Cech had arrived from Chelsea with a reputation for allowing few goals, but a header from Cheikhou Kouyate in the 43rd minute and a low shot by Mauro Zarate in the 57th unexpectedly beat Cech.
Arsenal toiled at Emirates Stadium, dominating possession but lacking ideas against a West Ham side that had been playing competitive preseason soccer in the Europa League under its new manager, Slaven Bilic.