I think you are cherry picking facts a little bit... he played regularly for BOLTON f*ing WANDERERS so he is somehow more ready to play in the Premier League even though Chambers trains with us every day and has for a year+ and HAS played with the team, in the Premier League, and before then was trained with one of the best academies in the country?
Anyway, all of these situations are data-poor, so of course you have to go with what data you have and realize it is easy to be totally wrong b/c of the lack of data, but to me Holding looks better in almost every respect, not just athleticism (In fact, that didn't really occur to me b/c they are both decent athletes) - but now that you mention it, Chambers looks prone to getting his own feet tangled when turned and his overall agility is relatively poor (reminds me a bit of Mertesacker that way).
Re: minutes, clearly Mustafi will get slotted in but I don't see Gabriel as a threat any more at all. He has been an awful signing imho. Holding will be 3/4th behind Kos and Moose and Mert will likely get phased out. Will be fun to see Holding and Mert in Cups, etc.