Akron RubberDucks
The Cleveland Indians selected Armstrong in the 18th round of the 2011 MLB draft. In 2012, Armstrong pitched for the Carolina Mudcats of the Class A-Advanced Carolina League. He was named a Carolina League All-Star. He ended the year with a 2.06 ERA. He missed the beginning of the 2013 with an injury, and had a 4.14 ERA on the season after he returned to health. In 2014, he pitched for the Akron RubberDucks of the Class AA Eastern League. He was named an Eastern League All-Star. After the 2014 season, the Indians added Armstrong to their 40-man roster. Armstrong pitched for the Columbus Clippers of the Class AAA International League in 2015, and was named to the Triple-A All-Star Game.