Stats from 14/15 season:
http://www.telegraph.co.uk/sport/fo...anchester-Uniteds-hardest-working-player.html
He isn't the greatest tackler, no one denies that, but he is an attacking mid and I don't think there is a huge difference in his defensive contribution between him and that of other player in his position.
As far as Costa goal is concerned, it's amazing how you are blaming Mata, when Depay lost the ball, Schneiderlin was out of position and I think it was Blind who was marking Costa.
As far as the Spurs game was concerned, everyone saw Mata wasn't able to cope up, should have been brought off a long time before. Also, Darmian was poor in positioning as well, so instead of laying all the blame on him, be a bit unbiased in his judgement