Per 36 minutes, he averaged 10 points very efficiently, whereas Krstic is averaging only a couple of points more, but less efficiently.
Per 36 minutes, Erden averaged 10.5 pts and 7.5 rebounds on .600 shooting. Krstic averages 14.5 points and 8.5 rebounds on .510 shooting.
So it's slightly more than a couple of points more. Also, given that people generally agree that Krstic is mediocre rebounder at best, then by extension Erden should be quite pitiful.
As far as being a foul machine is concerned, Erden was almost as bad as Krstic -- both average in excess of 5 fouls per 36. It seems that this is the case of "the grass is always greener on the other side" situation.
As for Murphy, he sprained an ankle in practice. So you can think of this as "Troy Murphy and his bum ankle"... or think of it as trading away Erden and his bum abs (he played less than 1 full game with the Cavs and is still out).