Senderos is off I'll bet. Djrourou and Hoyte aren't good enough. Wenger no longer plays Eboue at right back. So ... Arsenal have no real quality second choices at right back or central defense spots. Micah solves that problem. And 2nd choice Centre backs traditionally get long runs in the Arsenal side.
Does Micah want to be 2nd choice even if he gets played somtimes. Arsenal have been linked with Kompany today who would be a good choice and Ben Afra for the left wing.