Hahn is making his second appearance with Oakland this season after going 1-1 with a 3.00 ERA in two starts. He has since made one start with Nashville, throwing 5 2/3 innings of one-run ball.
Muncy, 25, was hitting .279 with six home runs and 15 RBIs in 37 games at Nashville, ranking fourth in the Pacific Coast League in walks. This is Muncy's first stint with the A's this season after making his Major League debut last year, playing 45 games.
Surkamp allowed four runs in 2 2/3 innings in his start Wednesday, taking the loss against the Red Sox. He is 0-3 with a 6.55 ERA in five starts in two stints with Oakland this season.
Cash Kruth is a reporter for MLB.com. Follow him on Twitter at @cashkruth. This story was not subject to the approval of Major League Baseball or its clubs.