Reading Fightin Phils
On March 27, 2021, it was announced that that Appel would be attempting a comeback. Assigned to the Reading Fightin Phils to start the 2021 season, Appel made his first appearance of his comeback on May 8th against the Erie SeaWolves. He was promoted to Lehigh Valley after going 0–1 with a 5.84 ERA over eight total appearances, including six starts, with Reading.