"He came here and continued his four days of work," Farrell said. "Even with [Monday's] off-day, he came out and worked. He'll travel back and make his start [Wednesday] at Pawtucket."
In his previous two appearances for Pawtucket and Double-A Portland, Masterson allowed four runs over 6 1/3 innings. Most importantly, he did not have any setbacks with the injury.
If Masterson continues to progress, Boston could be faced with a difficult decision.
With the a break in the schedule, the Red Sox reverted to a traditional five-man rotation as opposed to using six starters. Right-hander Joe Kelly remained in the rotation, while righty Steven Wright is working out of the bullpen.
If Masterson is deemed healthy, Farrell will have to decide whether to use him as a reliever or put him back in the rotation. Masterson is 2-2 with a 6.37 ERA in seven starts for Boston.
Farrell declined to speculate on when Masterson might return to the club.
"We'll see what [Wednesday] brings," Farrell said. "We obviously just went through one decision on the rotation. We'll see where Justin puts himself in the mix and forces another."
Todd Karpovich is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.