I seem to remember reading somewhere that days prior to the Peoples Volunteer Army went to Korea, Mao and Stalin had agreed Korea was a lost cause and it would be best not to get involved.
Alternatively you could have MacArthur acting more intelligently (ASB I Know) and halting the UN at the Chongchon River and building elaborate defenses in case the Chinese do come across before advancing to the Yalu. When and if the Chinese invade, the UN is ready for them and falls back to their defensive positions making a prolonged Chinese presence in North Korea costly and unpopular with the upper echelons of the Communist Party.
Sometime before World War II began, Eisenhower, who was MacArthur's aide
in the Philippines, was asked by a Washington socialite, her name escapes me at the moment, if he knew General MacArthur. This was at a cocktail party in the 1930s, I think. Eisenhower's reply was as follows: "Know him?
Madam, I studied under him!"

Meaning that MacArthur had a flair for the dramatic. He was an incompetent ass in the Philippines. According to Edwin P. Hoyt's book The Lonely Ships, MacArthur, and Thomas Hart, the last commander of the Asiatic Fleet, hated each other. MacArthur, the commander of the troops of Operation Downfall, was literally the last one to know about the atomic bomb. Truman feared that if he knew, he'd blow it for the Americans to end the war when we did. One of things that got him canned as Supreme Allied Commander in Korea was his insistence on
the use of nuclear bombs in Korea. When MacArthur was fired, Truman replaced him with Matthew Ridgway. He, btw, wasn't Truman's first choice.
His first choice to replace MacArthur was General Walton Walker. However,
Walton Walker, like General Patton, died in the same way as Patton-----in
a traffic accident.