Newcastle's fourth successive win ended Arsenal's unbeaten run of seven games in all competitions.
Alex Iwobi nearly doubled Arsenal's lead moments later with a stinging strike that forced a good save from Martin Dubravka.
The Lascelles incident did not distract from a poor showing from the Gunners in the second half, in which Matt Ritchie struck the game's crucial third goal to condemn the visitors to a fifth away defeat in a row in the top flght for the first time since 1984.More news: Watch launch of TESS planet-hunting mission April 16
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Calum Chambers had a great chance to restore the Gunners' lead before the break in very similar fashion to Lacazette's finish, but the full-back could only clip the outside of the right-hand post after stretching to meet Mustafi's header across the box.
The result sees Arsenal stay sixth and means they are the same number of points - 33 - above bottom-of-the-table West Brom as they are behind leaders Manchester City. There is a new name though with Joe Tillock getting a debut in the Premier League in midfield and Holding starting at the back.
Ayoze Perez equalized for Newcastle in the 29th.
It was nearly 3-1 a few minutes later when Kenedy was played through on goal by a great pass from Slimani but via a deflection from Mustafi, Kenedy'shot came back off the bar with Cech beaten.