Perkins is terrible. He's easily the worst big man in the Thunder's rotation this year and is being outplayed by a rookie (Steven Adams). Green is a solid NBA player. Perkins is not.
People around here tend to overrate Perkins because he started his career here but honestly he was never anything more than a guy who eats up space in the paint. He's never been a good scorer (he's too slow, he brings the ball down on rebounds, he can't shoot, he has bricks for hands, his career FT% is only 60%), he's not a very good rebounder (career average of 6.1 rebounds a game which is pretty pathetic for a starting NBA center) and his defense is overrated (he was only a good 1v1 defender and wasn't known for his help side defense). Frankly, I think the only reason he was on the Celtics' roster was because he could guard Dwight Howard.