William Saliba and Kiwior can manage the back even if Gebriel is injured, moreover, Arsenal can still play three defenders and have a perfect game since Everton did not have fast wingers that can give Arsenal problem.
The three man defence woukd easily favour them against Everton because Everton are not performing very well currently and would not be a problem for Arsenal to easily beat them but their problem cluld start when they face tougher opponents like PSV and Tottenham that have good wingers that would trouble their three man defence.
those two matches would be a big test for Arteta now he's currently facing an injury crisis from his defence line and I'll be suprised to see him win against PSV and Tottenham, i have a big doubt though and my prediction is that he'll win Everton, maybe draw PSV and lose against Tottenham because they're currently the toughest amongst those three they'll face next, well Arsenal is a big club and they could suprisenly win all three.