Stewart was originally selected by Cincinnati in the third round of the 2008 First-Year Player Draft before being traded to Toronto on July 31, 2009. He entered the 2010 season ranked as Toronto’s No. 1 prospect according to Baseball America and went on to finish eighth in the Eastern League with a 3.63 ERA that season. In 2011, Stewart entered the season ranked as Toronto’s fifth-best prospect, along with having the “Best Fastball” in the organization according to Baseball America.