I dont know about all of MLB, but for the Indians, the most underrated or under appreciated players IMO have been Brian Shaw, Casey Blake, Jake Westbrook, and Cookie.
Shaw was constantly chastised, but for five years was simply one of the best relievers in baseball.
Blake's ability and willingness to play any position needed made him more valuable than simply his WAR. You had to consider that his WAR should be compared to whatever black hole in the lineup we would have had if he had not been able to play that position, and play it well.
Just think how good he would look on this team. Need a 1B? Need a RF so Naylor could play first?
Westbrook was denigrated constantly, simply because he wasn't an ace, but he was a solid #2 SP.
Everybody loves Cookie, but he was over shadowed by Kluber. Cookie was an ace who was never looked at that way here.
Over a five year period...2014-2018... he was the seventh best SP in baseball.