I thought this was a really interesting article. It helps explain why teams dumping him get better even though his traditional box score stats look good.
It's based on all the cameras the NBA has installed in the arena. It tracks all plays the players was significantly involved in.
http://www.basketballanalyticsbook.com/2013/12/30/raptors-7-3-since-dealing-gay/

From the article:

Rudy Gay is incredibly inefficient on offense. In previous posts, we defined Offensive Efficiency with Player Tracking (OEPT). It uses the new player tracking statistics from NBA.com and SportVU to approximate the points scored per possession that the individual was significantly involved in. Of the top 150 NBA players in Net Possessions Terminated (NPT), Gay has the lowest OEPT. You can check the numbers with the sortable table in this post.

On the over 27 possessions per game that Gay was seriously involved in, his team was scoring 1.06 points per possession. Houston’s Chandler Parsons has approximately the same number of NPT, but his team is scoring 1.39 points per Parsons’ possessions. In the approximately 27 possessions per game, that’s a difference of about 9 points.

The old box score stats suggest Gay is a valuable player, but modern analytics disagree. So, when Gay was traded from Toronto to Sacramento a few weeks ago, it became a sort of test of the competence of the new methods of player evaluation.