I think we saw the answer as to why Pochettino wanted another CB, and specifically wanted Sanchez. We all thought we'd play 3 at the back with Alderweireld anchoring the middle just as we did last year, with Sanchez simply being a slight upgrade on Dier. But why spend 40m on a slight upgrade?
Poch wanted something totally different, and he needed pace to do it. Alderweireld and Vertonghen played split very wide and with their excellence on the ball were almost auxiliary wide defensive midfielders when we were in possession, allowing there always to be at least one free man for Dier to work with at the base of the diamond and their range of passing kept the wing backs involved while their width covered for them. This only worked because Sanchez in the center with his pace was able to snuff out any risk of a ball over the top or between the center halves.
Until now, we have not had a CB which allowed us to play like this. Sanchez pace dramatically changes the core tactics of our defence and that's what Poch obviously wanted. Watching for 15 minutes yesterday and it was clear that Sanchez wasn't a signing for the future or an expensive marginal upgrade, he is key to a change in the way we will play going forward that Poch had envisioned probably all the back to last season but couldn't execute with the players we had.
Also interesting how hard we went after Foyth as well. Obviously CB he must see as a really key position for how we operate.