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 discovery of the submarine has unveiled a few "inconsistencies." For example, how can one explain the fact that the sub was found where it needed to be searched for from the start?
When on a state visit to Singapore, Russian President Vladimir Putin promised to revisit the discussion of the 1956 Declaration between the USSR and Japan regarding the issue of the peace treaty with Japan
The TurkStream, which runs along the bottom of the Black Sea from Russia's Anapa to Turkey, will consist of two lines, each with a capacity of 15.75 billion cubic meters of gas a year