Originally Posted by BeanerschnitzelX

I don't think he left DT, he just collaborated with A7X for Nightmare. He's not a replacement for The Rev either.

Originally Posted by http://en.wikipedia.org/wiki/Mike_Portnoy

On September 8, 2010, it was announced that Portnoy had left Dream Theater. Portnoy cited conflicting interests as the reason behind his departure.[1][2][3][4] This has led to speculation that Portnoy will be joining Avenged Sevenfold full-time as a replacement for that band's late drummer, Jimmy "The Rev" Sullivan, but these rumors have yet to be addressed.