In a statement, a spokesperson for Tierney said that the actress' breast-cancer treatments would conflict with the show's production schedule. "She is deeply disappointed that she will not be able to rejoin such a highly respected, talented, fun and funny group of actors and wishes to thank Jason Katims and NBC for their patience and support."
In its own statement, NBC said, "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 will recast Tierney's role, according to a source, and Parenthood is still expected to premiere at midseason. Written and executive-produced by Friday Night Lights' Katims, the show is based on the 1989 Ron Howard film about a dysfunctional extended family.
Tierney, 44, had surgery this summer to remove a tumor from her breast. NBC had previously pushed Parenthood's start date to midseason to accommodate her recovery. The statement also said that Tierney's doctors remain confident that the outcome of her treatments will be positive.