In Al Horford and Paul Millsap, we had two of the top 30 players in the NBA. Neither can be called a superstar, but they were both underrated. Put those two with a bonafide superstar, and you have a championship team. Al just isn't a superstar. It doesn't just have to do with scoring. It's just that he did several things well, somethings just ok, but nothing great. His ability to space the floor is what made him so valuable, but at the same time, his lack of go-to offensive moves and rebounding really kept him from being elite. You might call him underrated, but the fact that he made a few all-star teams calls that into question, as does the huge contract he got with Boston. He's been valued very highly in his career, and deservedly.