NEW YORK -- Left-hander Henry Owens was optioned back to Triple-A Pawtucket on Friday with the vital mission of improving his fastball command.
The move came less than 24 hours after Owens walked six in just three-plus innings in a no-decision against the White Sox in Chicago.
To replace Owens on the roster, the Red Sox selected the contract of right-hander Sean O'Sullivan.
O'Sullivan will be available out of the bullpen on Friday. The Red Sox will need a starter to replace Owens on Tuesday night at Fenway, and O'Sullivan is a candidate.
"Henry needs to go back and look to command his fastball with more consistency," said Red Sox manager John Farrell. "He's got an outstanding changeup to get back into some counts and get him away from some damage. The strike-throwing is the priority here."
Owens made his initial stint in the rotation down the stretch last season, going 4-4 with a 4.57 ERA in 11 starts. Owens started this season at Pawtucket, and he was called up to fill in for the injured Joe Kelly. In three starts, Owens had a 5.11 ERA and didn't earn any decisions. Over 12 1/3 innings, he walked 13.
At one point, Owens was considered one of the best pitching prospects in the Red Sox's system. There's still plenty of time for the 23-year-old to reach that potential.
"He's doing what he can. I'm sure he'll be back to us at some point," said Farrell.
O'Sullivan made 13 starts for the Phillies in 2015, compiling a 6.08 ERA in 71 innings. The Red Sox signed the six-year veteran to a Minor League deal in the offseason, and he went 2-2 with a 3.00 ERA in five starts for Pawtucket.
"I'm super excited to be here, to be able to put the jersey on for real, instead of just Spring Training," said O'Sullivan. "It was 1 in the morning and I missed the first 12 phone calls they made to me. We finally got my daughter to sleep, so we were stone-cold out, and I just happened to hear my phone vibrating on the ground for the 13th time, and rolled over and said, 'Whoa, I'd better pick it up, because my manager has been trying to call me for 20 minutes.'"
Pablo Sandoval was transferred to the 60-day disabled list to make room for Sullivan on the 40-man roster. Sandoval underwent season-ending left shoulder surgery this week to repair a torn left labrum.