What's up with OJ Mayo's playing time?

I have followed OJ Mayo since he was in high school when he was praised as one of the best prospects ever. He was basically second for Rookie of the Year behind Derrick Rose, and last year even though his scoring was down 1 point, it could be attributed to the addition of Zach Randolph. So this year, I was wondering why he's been demoted to the bench for last 10 games or so, and have only been given about 20-25 minutes a game, which is pretty much the same amount of playing time as second unit guys. In my opinion he's still probably the third best on the team with lot of potential to improve, so why is he getting less and less playing time? So if anyone who lives in Memphis or follows Grizzlies basketball closely can give me some information on this issue.