The way it generally works is you first make some position coach changes, then it's either the offensive or defensive coordinator (or both), and finally you pull the trigger and fire the head coach. Generally, I wouldn't anticipate a change and Defensive Coordinator in the middle of the season, so perhaps letting the DC go after this season will be accepted as a no brainer decision. However, if the play on defense continues through the first half of this season, maybe you make the announcement then so you can go out and try and get a head start on recruiting for future seasons.
Thanks for posting, WK. Arizona would have to pony up about $10 Mil to buyout Sumlin's contract so I don't think he is going anywhwere.