» MauraTierney

Let's say first that Maura Tierney is pretty much irreplaceable.

As we already said in our 'Parenthood' pilot preview, Tierney was the MVP of a star-studded cast that includes Craig T. Nelson, Peter Krause, Monica Potter, Erika Christensen and more. But she was forced to drop out of NBC's midseason dramedy in order to battle cancer.

Now comes word that she might be replaced by Helen Hunt. And to that, we say: Nooooo! N to the O!

Bad news for Maura Tierney fans today: The actress has dropped out of NBC's upcoming dramedy 'Parenthood' to continue her treatment for breast cancer.

Tierney had surgery in July to remove a breast tumor, hoping to return to the show when she was recovered. NBC even moved the premiere date from fall to midseason to accommodate her, but now she and the network have agreed to part ways.

"While we are saddened that Maura Tierney won't be able to continue in her current role in 'Parenthood,' our main concern is for her quick and full recovery. Our thoughts and best wishes are with her," NBC said in a statement to Variety.

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What should you expect from the new TV shows coming this fall? Read our first impressions of NBC's 'Parenthood,' starring Peter Krause, Maura Tierney and Craig T. Nelson.

Premise: Struggling single mom Sarah moves her two teenage children back home to Berkeley, Ca., where she's welcomed back into the fold by her big, imperfect but loving family. Meanwhile, her three adult siblings all parenting issues of their own in this drama that is loosely based on the 1989 Ron Howard-directed film of the same name.

"I had a really great time there, but it's been almost a decade, and part of me is ready to move on."

When 'ER' wraps its 15th and final season in 2009, it will mark the end of Maura Tierney's 10th year on the show.

Going from obstetrics nurse to doctor, her Abby Lockhart has weathered alcoholism, a heartbreaking reunion with her bipolar mom (guest Sally Field) and relationship woes with Luka, Carter and Luka again.

Tierney shared with AOL TV her favorite 'ER' storyline, her thoughts on how Dr. Abby should exit County General, and more.