Manager Bobby Valentine will give Stewart at least one more chance to leave an impression on the Boston front office in a start that will likely come against the Orioles next Friday -- the first in a three-game series that could help decide the fate of the American League East or AL Wild Card race.
The former third-round Draft pick put up fair numbers with Triple-A Pawtucket this season, going 3-5 with 42 strikeouts to 14 walks and a 3.94 ERA over 11 starts. His career ERA in 100 1/3 Major League innings is 6.55.
Stewart was one of three additions to the Red Sox's roster made before Friday's game.
Third baseman Danny Valencia and right-handed pitcher Pedro Beato were also in uniform, and Valentine said both will likely be used in reserve roles whenever needed over Boston's final 11 games.