More than two weeks after a cyberattack, financially strapped doctors, hospitals and medical providers on Friday sharply criticized UnitedHealth Group’s latest estimate that it would take weeks longer to fully restore a digital network that funnels hundreds of millions of dollars in insurance payments every day.

UnitedHealth said that it would be at least two weeks more to test and establish a steady flow of payments for bills that have mounted since hackers effectively shut down Change Healthcare, the nation’s largest billing and payment clearinghouse, on Feb. 21.

But desperate providers that have been borrowing money to cover expenses and employee payrolls expressed skepticism at that estimate, worrying that it could be months before the logjam of claims and payments cleared up.

“We have nearly a three-week gap in cash flow,” said Brad Larsen, a psychologist and founder of Portland Mental Health & Wellness in Oregon, adding that the group had received only about 10 percent of its expected insurance payments. He said the practice had to borrow $300,000 to meet its first of two payrolls for the month. “It’s not good.”

In an apparent move to mollify some providers who had expressed disappointment at United’s earlier remedy of a loan program that offered stopgap payments of as little as $20 a week, the parent company agreed to issue advances. United announced that its insurer, the largest in the United States, would begin advancing payments to hospitals and doctors based on amounts billed before the cyberattack.

And given that Change manages one of every three U.S. patient records — amounting to 15 billion transactions a year, the cyberattack affected not only United’s clients but also those of many other insurers. That led UnitedHealth’s executive to recommend that they also offer advances. “To me, that is the quickest way to get money in the hands of providers,” Dirk McMahon, United’s president and chief operating officer, said in an interview. .

The depth of the cyberattack, which paralyzed billings and payments from the simplest prescriptions at a drugstore to the most expensive surgeries, has rattled the industry and government. Some have expressed concerns that the worst is far from over, fearing that the ransomware attack compromised patient data.

UnitedHealth Group has declined to comment on whether the information of its insured — whether financial or medical or whether through coverage at pharmacies, hospitals or clinics — had been hacked. Its only response has been to say that it is continuing to work with law enforcement agencies on an investigation of the attack. The F.B.I. and U.S. cybersecurity experts have been conducting an inquiry.

On March 1, a Bitcoin address connected to the suspected hackers, a group known as AlphV or BlackCat, received a $22 million transaction that some security firms said was probably a ransom payment made by United to the group, according to a news article in Wired. United declined to comment, as did Recorded Future, the security firm that initially spotted the payment.

“United has not been forthcoming about what information has been released to the hackers,” said Ed Tilley, a licensed clinical social worker in Charlotte, N.C. Among the information he typically submits for billing on the Change network is a patient’s date of birth and diagnosis. “If my patients’ identifying information has been disclosed, I feel an obligation to tell them,” he said.

Since the cyberattack became public, UnitedHealth Group’s stock has declined by 7.7 percent.

UnitedHealth Group said payments would start to become available only around March 15 and that it would begin testing and establishing the connections allowing hospitals and doctors to submit claims the week of March 18. But Mr. McMahon acknowledged that this time frame could change. “We’re in a very fluid environment,” he said.

“We’re hustling like crazy to bring these systems up,” Mr. McMahon said.

While most pharmacy transaction gaps appear to be resolved, he suggested that hospitals and doctors should continue to find workarounds. Yet for some providers, that has meant moving to Change’s competitors, which are now flooded with new claims and struggling to manage an increased workload.

“I submitted a few claims to the new system, which took me a couple of hours, and then I was like, ‘Where are they?’” and this bubble popped up saying, ‘No one can respond to you right now,’” said Angela Belleville, a mental health counselor in Salem, Mass. “I tried again yesterday and the system was completely frozen.”

Other major insurers have been largely silent on whether they would issue advances, as Mr. McMahon suggested, or offer other relief.

“It’s been crickets,” said Chip Kahn, the president of the Federation of American Hospitals, which represents for-profit hospitals. As the money from previously submitted claims begins to dry up, “you’re into the danger zone,” he said.

Smaller businesses, in particular, aren’t sitting on piles of cash that can tide them over while they wait for renewed reimbursements.

“We are past the two-week mark now, and people are starting to worry,” said Maggie Williams, the co-owner of Flourish Business Solutions, which advises medical practices on billing.

She says she has been getting calls from doctors concerned they may not be able to make payroll or that they will eventually have to stop providing services to patients in the coming weeks. “A lot of times, there aren’t reserves to be able sustain services or payroll,” she said.

In a statement, the American Hospital Association, a trade group, said, “Nothing in the announcement materially changes the chronic cash flow implications and uncertainty that our nation’s hospitals and physicians are experiencing as a result.” The group also said it would be “weeks — if not months — before our hospitals and other health care providers will be made whole.”

The powerful hospital lobby was among those who have been calling on federal officials to alleviate these pressures by accelerating Medicare reimbursements to providers, similar to the efforts made during the pandemic to tide hospitals and doctors over.

This week, the Department of Health and Human Services announced a series of steps, including trying to advance Medicare payments to providers. The department urged private insurers to do so also and called on private Medicare plans to relax or waive the much-criticized prior-authorization rules that make it more difficult for providers to be paid for care.

UnitedHealthcare also announced it would also relax its prior-authorization requirements for its Medicare Advantage policies until the end of March.

Beyond the news of the damage caused by the cyberattack, the shutdown of parts of Change Healthcare cast renewed attention on the consolidation of medical companies, doctors’ groups and other entities under UnitedHealth Group. The acquisition of Change by United in a $13 billion deal in 2022 was initially challenged by federal prosecutors but went through after the government lost its case.

On Friday, providers seeking advice or help from a human in customer support at Change Healthcare instead were greeted with a recorded message: “Due to unforeseen circumstances, we are unable to answer your call at this time. Please try your call again later. Thank you for calling.” And then the call was disconnected.