For all the attention given to forays from Amazon, Apple and Walmart into the healthcare space, traditional providers are more worried about the continued expansion of UnitedHealthGroup’s Optum into clinical care, according to an article in Bloomberg.

Unlike the more recent moves by potential outside disruptors, Optum has been acquiring practices for nearly a decade. It now employs or is affiliated with 30,000 physicians and will add on another 17,000 as long as its $4.9 billion acquisition of DaVita Medical Group is completed as expected later this year—giving it a physician workforce beyond that of for-profit hospital giant HCA and any other medical group owner in the U.S.

“This is obviously scaring the crap out of hospitals in many markets,” said Chas Roades, CEO of consulting firm Gist Healthcare.

With that reach comes more power to keep patients away from higher-cost hospital stays as well as limiting care to UnitedHealth’s narrow networks. While the company won’t say which markets it operates in, confirm its roster of employed physicians or detail how many patients come from UnitedHealth insurance plans, it also said it has no plans to shut out other insurers.

“We have been slowly, steadily, methodically aligning and partnering with phenomenal medical groups who choose to join us,” said Andrew Hayek, who oversees the care delivery operation at UnitedHealth. “The shift towards value-based care and enabling medical groups to make that transition to value-based care is an important trend.”

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