American specialist Dr. Richard Steadman, based in Vail, Colorado, was due to complete the cartilage operation Saturday.
"Ji was initially examined by our own physios but with these situations, if you have complications you send the player to a specialist," United manager Sir Alex Ferguson said.
"We either had to operate or let nature take its course. Because there was no clear reason for the injury, we knew it was bad enough for us to be concerned, which is why he has gone to America."
The Red Devils will assess Park's recovery in August, which means he will miss United's ongoing bid for three trophies and South Korea's Asian Cup campaign in July.
Park joined United in July 2005 from PSV Eindhoven.
The USA does not have a picture of the strong Russia, and the Americans will never allow Russians become strong. Sanctions show how obvious the conflict is
Fearing that peace might break out with the two Koreas talking to each other, Washington instructed South Korean President to keep the message about anything but peace
The head of the British army, Nick Carter, said that Moscow was capable of taking "hostile actions" against the United Kingdom and NATO much earlier than expected