Can't say I noticed any change. Remember how he pulled out of 50-50 tackle one game, costing you late goal and 3 points? Mata will never have the right mentality to be good defensively, it's always in the back of his head to avoid contact. Which is understandable of course given that he weights like 50 kilos, but it was never the case of him not working enough. He worked very hard in Chelsea too, but he just lacks that much needed tenacity in tackle.
You'll be suprised then. It's not about defensive tackling, but pressing, which isnt the same thing. He shouldnt be trying to make a last ditch 50/50, he should be harrying when the ball is lost, he did that excellently, not that LVG should have had him doing that down the right hand side, but thats beside the point