Piven Wasn't Sick, He Was Fired: Source

It may have been a pink slip, not a doctor's note, that prompted Jeremy Piven's premature exit from Speed-the-Plow. Diva behavior befitting that of his Entourage persona got him booted, Broadway insiders tell E! Piven—who cites mercury poisoning—wanted out, they claim, because he couldn't handle the nightly grind. He'd arrive minutes before showtime and send out his understudy if the audience size wasn't up to snuff.

"Jeremy was not fired from the show," maintains his rep, who provided a doctor's number to confirm his high mercury count. "This show has been his number one priority since he started rehearsals."
(More Jeremy Piven stories.)