Moody's Investors Service downgraded Lowell General Hospital's debt to Baa2 from Baa1 and removed the rating from under review.
The outlook is stable at the lower rating level, Moody's said.
According to analyts, the merger of Lowell General Hospital and Saints Medical Center, also of Lowell, Mass., results in "materially weaker balance-sheet measures, weaker profitability for the combined system during the early years of the integration, and transition risks."
Citing its stable outlook, Moody's said that long term the merger would increase market share, enhance efficiency and keep external acute-care competitors from entering the area's health care market.
Lowell is located about 35 miles north of Boston.
