In any case, Di Maria was shit last season. His stock has dropped since we bought him. Those expecting to earn back every penny are deluded.
The state of this I see time and time again. Not getting at Dante in particular here, tend to agree with a lot of what he writes, but the general attitude to Di Maria reeks of ignorance from a lot of fans.
Statistically Di Maria performed exactly as he has done all of his career - assist machine and a few, but not many, goals. People see the price tag and expect miracles from a man whose career has never shown he can do it. Nobody appreciates what he has done and what he does do. Most clear cut chances created and 2nd most assists. In a team that struggled for goals all year round.
Now, I completely concede that not starting and losing form toward the end of the season isn't ideal but he also came off the back of his longest ever season, followed by a World Cup and facing a Copa America coming up too. It's entirely understandable in a new league to dip form at some point in the season, never mind the intense pace and physicality in the English game. Or the change in tactical nature heavily imposed by Louis.
Oh and fwiw, man of the match in the Champions League final, widely regarded as Argentina's player of the World Cup and this Copa America.
Far from "shit" and far from his stock dropping. We're not talking a one season wonder here. This is a player of world class pedigree. Simple.
Now to answer the OP, I think Mata stays there to offer a counter balance to Memphis on the left. As he develops we will be inclined to relieve his defensive duties to maximise his offensive threat. Parallels in attacking systems tend to struggle when you've got a Memphis style offensive focused player. In 07-09 we had Tevez and Rooney to offset Ronaldo for example.
Could see Pedro working as his attitude, work rate and eye for goal are all very good. His passing is often very safe however. His best work is off the ball movement to get into positions.