If this was planned in advance, why was there no immediate replacement?

The Microsoft reorganization was first leaked by the media on June 2nd.

This means that it most likely was initially discussed and communicated internally months before.

I wouldn't be surprised if Don communicated this to Ballmer and Co. well before E3 as a transition period. His last day is July 8 and a position like Microsoft Entertainment Division CEO doesn't just give 2 weeks notice to the whole company.

Because the re-org hasn't been communicated yet. Once they announce the company shuffle it will be clear where the Xbox division fits in and who will lead...

ballmer is working on changing a lot at MS.... he wants to merge several department into one.... and apparently hasn't been consulting many people for his plan.... I'm sure he was aware of mattrick's intentions.... but the "extreme makeover" of MS is not an easy task and we will probably see it happen sooner than latter.... but from what I picked up it should be win8 division (comprised of everything windows, WP, Win and win RT and more) a enterprise department and hardware with XB, surface and who knows maybe an in house phone division if you sum it up of course.... well not clear yet but he wants to streamline everything to have united ecosystem in the future.... and we don't even know if he will act upon it but that's his vision and what has transpired in the financial world lately....
me I'd love to see one of those chicks top exec take a big role in the XB brand future... :P

I suppose it's for the best, anyway. His management style will match Zynga perfectly and they can all dance the merry dance of bankruptcy together.

I think he got fried because of all his bull that he said

Well I don't blame him, he got a lot of stick for how he handled the xbone.

Lol

That's the kind of purposely leaked rumor you'd expect to hear.