There's a difference between giving players time and accepting awful play from them, don't expect them to excel but do expect them to be competent at least and not make basic errors.
It isn't Wenger's fault he doesn't perform (well maybe partly is as he doesn't motivate players), Wenger picks him but if Koscielny can't perform it's down to his lack of concentration or inadequacies.
Granted Wenger shouldn't pick him, but since he does the guy should at least be competent and avoid own goals like the one we saw at the end, that was so casual it's untrue.