Good is an adjective, and it modifies nouns. Well is an adverb, and it modifies verbs. 'The US didn't play as well as South Korea' is correct, because you want to modify the word 'play', which is a verb. However, people often use 'good' as an adverb even though it's not technically correct. It's used like that when you want to sound uneducated or informal. 'Me talk good', or 'You did good, kid' are examples. 'Bad' is also an adjective, so 'The US didn't play as bad as South Korea' is also not technically correct, although you might still hear it spoken. The word you're looking for is 'badly'.

Hi Vitor, the correct way of saying this is: The U.S. didn't play as WELL as South Korea. Compare these two sentences: It was a good game. Our team played well. Good is an adjective and well is an adverb. Considering this, you need an adverb in the other sentence too: The U.S. didn't play as BADLY as South Korea. The comparative of the adverb badly is worse, and the superlative is worst. So you only have to decide whether you need an adjective or an adverb and then you can't go wrong.