The news broke on the website of the Florence River Civic Center in South Carolina, where the pair were scheduled to play on Thursday, Oct. 15.
"We have just been informed that Willie Nelson has a medical issue and will be rescheduling the first half of his tour dates," wrote Nick Hooker, Senior Marketing Manager for SMG Florence Civic Center, in a statement.
According to the statement, Nelson and Haggard have rescheduled Thursday's show for April 6.
"This is all news to us at the moment and we're still getting details on Willie's medical concern, said Hooker. "We've been told this is not life-threatening, but will require several weeks to fully recover."
According to Nelson's camp, the remainder of the tour will continue as planned. "I have no additional details but it is only the first three dates," said a rep for Nelson, according to Taste of Country. "The rest of the tour is on schedule."
We'll keep you updated as we learn more about the situation.