Arsenal manager Arsene Wenger felt goalkeeper Lukasz Fabianski could perhaps have done better for the goal which earned Newcastle a 1-0 win.
Although the Gunners twice hit the woodwork through Cesc Fabregas' deflected free-kick and an angled drive from Theo Walcott at the start of the second half, Wenger's side were well below par.
A miserable afternoon was compounded by the dismissal of centre-back Laurent Koscielny in stoppage time for a professional foul as Arsenal suffered a second successive defeat, following their midweek Champions League loss away to Shakhtar Donetsk.
Wenger said: "He didn't have a lot to do and I think he was a bit too confident to take this ball.
"It was not a technical mistake, it was a problem of timing and he thought maybe he wouldn't be challenged."
Source: PA