Eric Church has cancelled a huge concert this weekend so he can watch his beloved UNC basketball team play Duke in the Final Four ... and the country music superstar's fanbase is not happy with his decision.

Church announced the cancellation in a message to his supporters Tuesday -- explaining he wants to watch the Tar Heels game on Saturday night so badly that he won't be able to perform at the AT&T Center in San Antonio.

Church called the move "the most selfish thing" he's ever asked of his fans in a statement to them ... but he implored them to understand because the game just means that much to him.