As we reported last week, the lovely Kaley Cuoco was set to guest star on Grey's Anatomy, only to suddenly disappear from the cast a few days ago.

Well, EW has cracked the case of the mysterious vanishing guest star.

Although the actress' rep says an unforeseen "publicity commitment" forced The Big Bang Theory star to back out of a top-secret appearance on Grey's Anatomy's May 14 season finale, multiple sources say it was her current employer, CBS, that nixed the deal.

An CBS spokesperson couldn't be reached for comment, but one insider explains, "They didn't want one of their biggest stars appearing on one of ABC's biggest shows."

As a result, Grey's Anatomy has instead tapped former OC starlet Shannon Lucio (she played Caleb's illegitimate daughter, Lindsay) to fill Kaley Cuoco's shoes.

Still unknown: Who the heck is she playing?

Some speculated that, based on Cuoco's resemblance to Grey's Anatomy star Katherine Heigl, perhaps the character is a relative of Izzie Stevens.

That possibility may be less likely with the casting of Lucio... or is it?

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.