Optum, a business unit of UnitedHealth Group, has agreed to purchase Steward Health Care's struggling physician group. The group, Stewardship Health, spans nine states and includes primary care and other clinicians in addition to a physician contracting network.

The acquisition will further expand the size of Optum, which already has more than 90,000 employed or affiliated physicians and another 40,000 advanced practice clinicians serving tens of millions of people. The Massachusetts' Health Policy Commission (HPC) has 30 days to review the deal or request more information from the companies, which said the sale could close during this year's second quarter.

"This is a significant proposed change involving two large medical providers, both in Massachusetts and nationally, with important implications for the delivery and cost of health care across Massachusetts," said David Seltz, executive director of the commission. "Details of the proposal will be reviewed by the HPC to examine potential impacts on health-care costs, quality, access and equity. The sale cannot be completed until after the HPC's review and any concurrent review by state or federal antitrust authorities."