According to The Hollywood Reporter, production on "Parenthood" had already been delayed until November, but with Tierney's treatment extending later than initially anticipated, NBC is moving forward to recast her role.
"Ms. Tierney and her doctors remain confident that the outcome of her treatments will be positive," Tierney's spokeswoman told the trade paper. "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."
"Parenthood" had originally been scheduled to premiere this month on Wednesday nights as one of the anchors of NBC's fall, but the network swapped the drama, based on Ron Howard's 1989 film, with midseason drama "Mercy."
The series stars Peter Krause, Tierney, Erika Christensen, Dax Shepard, Bonnie Bedelia and Craig T. Nelson.