To be honest, I don't think Di Maria was anything like Robben.
Under Mourinho at Madrid he provided some creativity, but what I felt he mainly brought to the team was hard work and defensive diligence to compensate for Ronaldo on the other side of the pitch - a real shadow of the player he could be.
This is why I included him - he provided something to his team on the right, so it wouldn't be inaccurate to say he was effective there. Compared to his impact on a team in a more favourable role, however, it's a poor position to deploy him in. Hence the grouping with Sanchez.
That last season at Madrid, Di Maria was their best player. Better than Ronaldo. It's easy to dismiss it as a one-off, freak season, but that was the first time he had been played in an ideal position since earning his move to Real on the left side of a midfield diamond for an utterly brilliant Benfica side. And he picked up where he left off at United initially, before it all tailed off for most likely a variety of reasons but the change of formation and him being shifted out to the wing certainly was a contributor.
He has played the majority of his career out of position, and it's impossible to know just how highly he could have been thought of under more advantageous circumstances.