BOSTON -- Going for upside, the Red Sox have brought in another left-handed reliever. General manager Theo Epstein announced Thursday night that the team had acquired 25-year-old Franklin Morales from the Rockies for cash or a player to be named later.

The deal was announced shortly after the Sox's 4-3 walk-off win over the Tigers at Fenway Park. Left-handed reliever Hideki Okajima, who's never played for another Major League organization, was designated for assignment to make room on the 40-man roster. Okajima had been with the Red Sox for five seasons.

"This is a guy who was one of the top prospects in all of baseball a few years ago," Epstein said of Morales, "and he's been a little bit erratic with his strike throwing, but there still is plenty of upside there."

Epstein said he got word a few days ago that Morales was available and felt the cost was low and the potential reward high. Morales, a Venezuela native, is 0-1 with a 3.86 ERA in 14 innings this season. He's struck out 11, walked eight and allowed 10 hits, including a pair of home runs this season. He was roughed up in his two most recent outings, allowing a combined three runs in one inning.

Epstein said he was hopeful Morales could join the Red Sox on Friday. The Rockies played the Phillies in Philadelphia on Thursday and then headed to Milwaukee for a series that starts Friday. Morales did not pitch Thursday.

The Red Sox's bullpen is thin because relievers Alfredo Aceves and Tim Wakefield have slid into the rotation to replace the injured John Lackey and Daisuke Matsuzaka. Morales is a full-time reliever, but he also has experience as a starter, last making a big league start in 2009. Boston seems to have him ticketed for relief duty.

Lifetime, left-handed hitters have a .185 average against Morales, and right-handed hitters a .272 average.

"He's going to take that second lefty in the bullpen spot and see if we can capture some of that upside," Epstein said.

Okajima had not pitched in 10 days, last throwing against the Twins on May 9. He was 1-0 with a 4.32 ERA in 8 1/3 innings over seven appearances this season. Okajima started the season at Triple-A Pawtucket, losing a tight race in Spring Training to be the team's southpaw out of the bullpen.

On Thursday night, he sounded resolved to the idea that his days with the Red Sox were over.

"I started the season down in the Minor Leagues, so I know I had to regain the team's confidence in my pitching," Okajima told Japanese reporters according to team media liaison Mikio Yoshimura. "It is my first time in this situation, so I'm not sure of what happens next."

"Having re-signed with Boston during the offseason, it is disappointing that this is happening, but signing here was not a mistake. I am very grateful to the opportunity the Red Sox have given me over five years.

The Red Sox have 10 days to trade Okajima, reassign him to a Minor League affiliate or release him. Okajima was signed to a one-year, $1.75 million deal this offseason.

"We weren't using Oki all that much the way things have evolved lately, and Morales is somebody with significant upside if we can get him to repeat his delivery and throw strikes," Epstein said. "He can be tough to hit."

Okajima became expendable in part because of left-hander Rich Hill's emergence. Hill hasn't allowed a run in 10 appearances with the Red Sox dating back to last season.

"I think Rich's emergence made us more comfortable making this deal," Epstein said. "Rich has been throwing the ball well all year down in Pawtucket and has gotten off to a nice start up here."