Forest have got quite a bit of financial restructuring to do in the next few years, otherwise they haven't a hope in hell of complying with the new financial rules.
From what I've heard Billy's departure was hastened because he had another falling out with Doughty when he found out how much money he was being given this season. Either McClaren will be given the same amount, or he'll be given more as a 'last stab' at going up.
If Forest don't go up this season then it'll be cost-cutting ahoy next season onwards.
Look's like McClaren's the last great gamble that Forest can afford.