London - With Olivier Giroud sidelined by a broken tibia, Yaya Sanogo started up front for Arsenal at promoted Leicester and he played a central role in the visitors' 20th-minute opener.
The French striker could not find a way past Kasper Schmeichel from Santi Cazorla's dinked pass, but Alexis Sanchez followed up to slam home his first league goal since signing from Barcelona.
Arsenal's lead was to last only two minutes, however, as Jeff Schlupp motored down the left wing before delivering an inviting cross that Leonardo Ulloa powered past Wojciech Szczesny with a firm header.
Arsenal centre-back Laurent Koscielny hurt himself in a clash of heads with Schlupp when the score was still 0-0 and he made way for Calum Chambers shortly after Ulloa's goal.
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Leicester came closest to scoring a winner in the second half, but Ulloa clipped an effort into the side-netting after cutting inside Chambers while Szczesny produced a diving save to thwart substitute Jamie Vardy.
"It was tough. Leicester played very well and defended with a lot of heart," Arsenal manager Arsene Wenger told Sky Sports.
Asked if he would look to bring in a replacement for Giroud before the transfer window closes on Monday night, he added: "We are active in the window. We are working hard to find a solution."
The result left Arsenal four points below leaders Chelsea in seventh place in the table, while Leicester moved up to 15th.