Really hard to say if a player is underrated, unless you've seen him play with your own eyes. I started following basketball in the mid 90's. The first name that comes to mind is Tim Duncan. Imo, he's a GOAT candidate. Obviously, his career averages aren't worthy of a GOAT candidate.
The Spurs during the Timmy era were a very special team. If you ask me, most of their star players at the time were underrated. I mean, Manu Ginobili is definitely underrated! Call me crazy, but I truly believe that Manu at his peak was contributing more to winning (/was helping his team more) than megastars like Kobe-Iverson or superstars like Harden, Westbrook, Carmelo etc. Or how about Kawhi (as a Spur)? The guy was good enough to win Finals MVP, yet he was averaging 12.8 points per game during that season! At the end of the day, the Spurs players were asked to sacrifice their personal numbers for the good of the team. No wonder most of their stars were underrated (at least imo).
Only exception I can think of is Parker whom I believe is overrated, but that's a whole nother story.