Bill Russell won four MVPs while never averaging 20 points a game. Why?
Because he was the greatest defensive player in the history of the game.
Russell does not have the benefit of defensive boards or blocks to justify his place as the greatest defensive player (though, unofficially, it is suspected that he often blocked five or more shots a game; averaging that in a single season would put him in Mark Eaton territory).
However, his total rebounding numbers are impressive...he finished in the top three in boards in each of his 13 seasons, and is second all-time in total rebounds and rebounds per game.
And the stat that really justifies his defensive prowess is defensive win shares.
Russell has 133 defensive win shares. No other player has 100, even with the benefit of at least two or three extra seasons.
To put it another way, Bill Russell contributed 10 or more defensive win shares in a season six times. Only one other player (Wilt) has done it in the course of the season.
Russell's career average defensive win shares per season is 10.1. He holds the single-season record for defensive win shares with 16, and has lead the league in defensive win shares more times than anyone.
The greatest defensive player.