He finished the 2011 season with a 4.50 ERA, 15 strikeouts, a 0–2 record, and a 1.72 WHIP. On April 2, 2012 Harrell was declared a starting pitcher in the team's Opening Day rotation. In his first start of the season, he threw seven scoreless innings and allowed three hits while striking out four against the Colorado Rockies. Harrell also got his first major league hit, a bunt that hugged the third-base line and stayed fair. On June 27, 2012 he recorded his first shutout and complete game in a 1–0 win over the San Diego Padres. Not since Taylor Buchholz in 2006 had an Astros rookie pitcher recorded a shutout. Harrell finished with team bests in wins (11), innings pitched (193.2), ERA (3.76) and in WHIP (1.35). In 2013, Harrell pitched poorly throughout the season. At one point he was demoted to the bullpen but was placed back in the rotation by the end of the season. Harrell finished with a league high 17 losses accompanied by a 5.86 ERA in 36 games (22 starts). He also finished with a league high 88 walks. On April 16, 2014, the Astros designated Harrell for assignment. He was outrighted to the minor leagues on April 22. Arizona Diamondbacks On April 28, 2014 the Astros traded Harrell to the Arizona Diamondbacks in exchange for a player to be named later or cash considerations. The Diamondbacks released Harrell on August 24, 2014. His stats were poor, after starting out well in his first few starts in Reno, Harrell declined gradually over the course of the next few months. In over 100 innings pitched, Harrell had 77 walks against 67 strikeouts and registered a WHIP of 1.80 in 22 games for the Aces. LG Twins On November 25 2014, Harrell signed with the LG Twins. Detroit Tigers On March 7, 2016, Harrell signed a minor league contract with the Detroit Tigers. Harrell appeared in six games between the AAA and AA minor league Detroit Tigers system. Atlanta Braves Harrell was acquired by the Braves on May 21, 2016. On July 2, the Braves called him up to make a spot start. Harrell pitched six innings, yielding one run on five hits in his first major league start since April 15, 2014, and earned his first win since August 14, 2013. Texas Rangers On July 27, 2016, the Braves traded Harrell and Darío Álvarez to