Completely for the concept of a defensive midfielder, as we've not really ever had a good one (or really used the role correctly), and we all know that our defence is historically leaky as an old boat and we can't close down players encroaching on the box looking to shoot. A decent DM will either stop those players getting into those areas at all (cut ball out or cut option off), or close down the player to free the CBs up to back up and watch for runners and one-twos.
I do wonder though, however great Dier is playing in this role, and he is playing great, whether it might be Jan that could really excel here in the longer term. He's good with the ball at his feet and bringing the ball out of defence, and prone to a critical slip-up when the last man. I see Dier as our CB for some time coming if he can continue this trend of improvement, and I think we're probably grooming him for that role after this spell at DM a-la King (even if that was never the intention with King, it certainly helped him). Maybe we give him some utility experience for a half season and then try to swap him and Jan and...see how that goes. I think Vertonghen would make a great DM. Good with the ball, and can read the game well but wouldn't be having to make the critical last-ditch tackles with a lack of pace. Dier can and does make those tackles (another at the weekend from DM), is quicker across the ground and more assertive and assured. Jan is no leader, but he wouldn't have to be. Dier can be. I think Jan would make a great clean-up, break-up, cover man, and probably better at turning play around than Dier after recovering/intercepting the ball.
But...who knows. I had a couple of beers watching tonight's shower of brick, so not sure what I know!