He's been a monster in every game this season in which he played closer to the middle and had Arda/Griezmann to link up with. Some of the passing sequences that trio come up with are sexy as hell. In the 4-3-3 he has way more passing options and his class shines since Atletico can actually control the ball better and play their versatile game. What a player.
Still wouldn't feel good about poaching him on a transfer. We have Rakitic now as a more than capable stop-gap and Samper/Kaptoum coming down the line (both with a very high capacity to succeed unlike the others who have greater personal obstacles like Deulo or Adama). And if a transfer really does end up being imperative, as said before, Gundogan would be a lot cheaper.