If we buy Vidal and Montoya doesn't leave in January, we'll be set. Even with Alves gone and Adriano possibly leaving.
Alba / Mathieu / Vermaelen at LB
Vidal / Montoya / Douglas at RB
That's more than enough depth, in numbers, at least.
And Mathieu and Vermaelen get to rotate both for LB and CB, which is great for them.
The only question is whether Montoya will leave if we get Vidal for the RB spot. It sucks, really. If we hadn't bought Douglas, Montoya would have less reason to leave, and would be more OK with rotating with Vidal, who is not as much of a competition for him as Alves is/was. Douglas was the worst transfer ever. Even with Montoya being better than him, he's still a reason for Montoya to leave.