Appel, who was the top choice in the 2013 MLB Draft by the Houston Astros, has not been able to break into the big leagues after experiencing a ton of ups and downs during his five seasons in the minors.
After a decent 2013 campaign in both Class A and Class A short season, Appel missed the beginning of the 2014 season with an appendectomy, then struggled mightily with Single-A Advanced Lancaster, posting a 9.74 ERA in 12 starts.
In 2015, Appel reached Triple-A Fresno and played in the All-Star Futures Game, only to be traded to Philadelphia in December in a seven-player deal in which the Phillies dealt Ken Giles to the Astros. In 2016, Appel underwent surgery to remove bone spurs in his right elbow, then came back to throw 38 1⁄3 innings before shutting it down in June of that season.
Last season, Appel had another tough year, going 5-4 with a 5.27 ERA and a 1.76 WHIP in 17 starts for Triple-A Lehigh Valley. Appel has logged 375 innings and owns a 5.06 ERA since debuting as a pro.