I don't understand why he's resigned today, its unlikely that anyone else can change their form now, and they're not going to get a full time appointment with 3 games to go. For me he's taken the cowards way out, resigning before it can be said he was the manager when they were relegated.
I'm not sure he ever had much hope there, a bad appointment by the club, I expect their new owner wanted a certain style of play which was why they brought him in, but the championship is not the place to try that, you need proven managers at that level, even Benitez (thought of as a world class manager by many) is struggling to get Newcastle over the finishing line, despite the budget they've had.