Johnson is slated to start Saturday for Triple-A Pawtucket.
A line drive caught Johnson in the leg during his Aug. 16 start, forcing him from the game after just one inning, but he returned to action five days later. He figures to make two more starts (including Saturday's) before rejoining the big club in September, but it's uncertain if Johnson will start at all for the first-place Red Sox over the season's final month. Johnson recently battled some shoulder fatigue and an assignment to the bullpen would be theoretically help preserve the shoulder.