Like I've explained a few times, and if you ever watched us play in the mid 00s you'd know that we always had one winger who would be involved more in midfield while the other went forward and turned us into a 4-3-3ish attack. When Giggs and Ronaldo played, it was Giggs. In the CL final, it was Hargreaves. It is that role Di Maria has here, where he will be more the supplier and not the one hugging the touchline, and often joining the midfield.
There are plenty of configurations where Di Maria's left midfield role would be beneficial to the team. Him coming inside can allow Koke to go outside on his favoured flank, or or similarly Rooney can occupy that side while someone like Saul supports the attack down the middle. And there is Asamoah, who would easily provide the width when he comes inside.
It's a role that mirrors his performances at Madrid. he wasn't a central midfielder in that team, he pretty much provided width when needed. Heck, just watch this. This is the reason he's known around the world for the most.
Scroll to 4:18.