Troubled hospital operator Steward Health Care, grappling with a financial crisis that's engulfed its eight Massachusetts hospitals, has moved to shore up its finances by striking a deal to sell its nationwide physician network to insurance giant UnitedHealth's Optum Care unit.

The proposed sale of the doctors group, called Stewardship Health, is part of Steward's plan to bolster its national system of 33 hospitals after the Dallas-based company last year fell behind in paying bills and rents.

On Tuesday, a subsidiary of Optum, Collaborative Care Holdings, LLC, filed notice with the state's Health Policy Commission over the sale. A price tag was not included in documents filed with the state, and Optum officials did not immediately respond to comment.