Feb 19 (Reuters) - UnitedHealth Group UNH.N said a mistake at the hospital was behind the phone call that led to a surgeon being pulled from the operating room to defend a patient's inpatient stay, a move that was eventually criticized by billionaire Bill Ackman.

UnitedHealth said that it had already approved the treatment and overnight stay for the surgeon Elisabeth Potter's patient before the call. Its representative called the hospital to discuss a request for a separate inpatient stay.

"Our representative asked to speak to the nurse caring for the patient. He did not ask to be transferred to the operating room department, did not ask to speak with the physician while she was in surgery and never asked the doctor to leave surgery," the company said.

The hospital where the incident took place was identified as HCA St. David's Medical Center in Austin, Texas, according to Bloomberg News, which first reported on the matter.

Earlier this month, Potter posted on the social media platform X, saying she was called out of an operating room to justify her patient's care to the insurer.

On February 17, Pershing Square's CEO Ackman said that UnitedHealth sent a defamation letter to Potter and added that he offered to pay her legal bills.

Ackman had previously posted on X that he would take a short position in the healthcare conglomerate, in a now-deleted post.

"If I still shorted stocks, I would short UnitedHealth," Ackman said in his post on February 17, adding that UnitedHealth's "profitability is massively overstated due to its denial of medically necessary procedures".

Shares of the health insurer were up 2% in afternoon trade.

(Reporting by Christy Santhosh in Bengaluru; Editing by Mohammed Safi Shamsi)

((Christy.Santhosh@thomsonreuters.com;))