Di Maria this season: 10 starts, 3 off the bench, 2 goals, 1 assist.
Di Maria last season: 22 starts, 10 off the bench, 7 goals, 6 assist.
There's probably a reason he's not rated as highly as you guys think he should be.
9 goals and 7 assists in a season and a bit aren't unbelievable stats for a winger who plays for Real Madrid. I'd say there's a reason they were looking at moving him on in the summer.
The most underrated player would most likely be a midfielder/defender. Just going from the PL (I don't watch enough of other leagues) players like Mertesacker, Nastasic and Evans are ridiculously underrated. Mertesacker especially, who most people would try tell you is a liability.