In his last three OKC years his per-36 scoring averages were 14.7, 14.8 and 14.8. He averaged 14.9 with Boston.
His PERs in his last three OKC years were 13.8, 12.9 and 12.9. His PER with the Celtics was 12.9.
Rebounds? 5.8, 5.3 and 5.4. Then, 5.1 with the Celts.
Assists? 1.6, 1.6 and 1.8. Then, 1.1 with the Celts.
Pick any stat you want, and he has performed at the exact same level for four years now. He's reached his potential.
The only hope I see, and it's a long shot (no pun intended), is for him to become a better 3 point shooter. He is really mediocre (30%), but did shoot 39% in his second year.
If he can shoot 38% or even 35% from 3, he becomes a starting-caliber player in the NBA comparable to Nic Batum.
If not, he is a career bench guy.