To make room for Ross on the roster, the Red Sox optioned rookie right-hander Ben Taylor, who posted a 1.69 ERA in five outings during his first stint with the team.
Taylor helped save the rest of Boston's bullpen when Steven Wright was knocked out in the second inning of Wednesday's 12-5 loss to the Orioles, going 3 2/3 innings and allowing one run.
"He'll be back," Red Sox manager John Farrell said of Taylor. "At some point he'll be back, whether it's a situation much like a game last night where rosters have to be adjusted, whether it's performance or injury, he'll be back. He shows a lot of ability with his fastball."
Taylor came to camp as a non-roster invitee and earned a spot on the Opening Day roster. He was a seventh-round selection of the Red Sox in the 2015 Draft.
"I think the thing that stood out to all of us was his poise and mound presence," said Farrell. "A lot of strikes, and the fact that he's showing us the ability to go three, four innings, that's a key piece to the bullpen. He impressed and has since the first day of camp through last night."
Ross, who hasn't pitched yet this season, joins a bullpen that includes two other lefties in Robby Scott and Fernando Abad.
"With Tampa [Bay] coming into town [Friday], where they've got as many as five left-handers in that lineup, we know that we're going to need the depth from the left side," said Farrell. "And here's the thing, prior to last night, we've had 11 pitches thrown by left-handed relievers. It's just a function of who we've played and the lineups we're going against. But now that the schedule starts to turn, there's going to be more opportunities for all three of those guys."
• Lefty David Price's next side session on Saturday will be about 45 pitches, and he'll simulate innings by sitting down about halfway through. He also might mix in some breaking balls, which he didn't do in his first two side sessions. There's a chance the next step after that could be facing hitters, be it in a simulated game or live batting practice against teammates. Price is coming back from a left elbow strain he suffered on Feb. 28.
• Third baseman Pablo Sandoval was out of Thursday's lineup, marking his first day off. Aside from two homers, Sandoval is off to a slow start, slashing .133/.206/.367.
"I think for Panda, just a little bit of a breather," said Farrell. "I think he's shown a tendency to be even more aggressive than he normally is where he's expanded the strike zone a little bit maybe too regularly."
According to Statcast™, Sandoval, who led MLB by swinging at 49.9 percent of out-of-zone pitches two years ago, had trimmed that to a healthier 33.1 percent for the 2017 season entering Wednesday.