Shouse was part of the competition for one of the final two spots in the bullpen, but he proved not to be a fit based on ultimate roster composition.
"He had such a good spring," said Francona. "We asked him to come in and compete, and he did a very good job of that. And statistically, he had a tremendous spring. Saying that, after our meetings and the construction potentially of our bullpen, it's probably not set up [for Shouse to be a fit]. You start looking at left-on-left, which is what his strength should be. I just don't know that we were going to do that as opposed to having a [Daniel] Bard or certainly a [Hideki] Okajima or even a [Manny] Delcarmen or Ramon [Ramirez]."
Those four relievers Francona mentioned, as well as closer Jonathan Papelbon, are definites for the Opening Night roster.
Boston traditionally goes with a 12-man pitching staff, leaving two spots open between several candidates -- a list that includes Boof Bonser, Alan Embree, Joe Nelson and Scott Atchison.
Embree was brought on board as a potential second lefty behind Okajima a week ago, which lessened the need for Shouse. Embree is less of a lefty-on-lefty pitcher than Shouse.
Shouse took the news with professionalism, shaking hands with several players before exiting the clubhouse.
Because teams always seem to be looking for lefties, he figures to surface somewhere else soon.
The Red Sox hoped that giving him the news with a full week left in camp would increase his chance of finding work elsewhere.
"Once we decided how we felt, we thought it was best to tell him, because he has had a good spring and I do think he will pitch in the big leagues, so it will give him a chance to go do that," Francona said. "To hang him out to dry until the end, it's not a good situation for him. It's a very difficult day. But it doesn't help any by keeping him here."
Shouse, 41, has made 467 appearances in the Majors, posting a 3.72 ERA. He spent last season with the Rays and was signed by the Red Sox as a Minor League free agent over the winter. Shouse did pitch seven games for Boston in 1998, notching a 5.63 ERA.
The Red Sox also optioned left-hander Fabio Castro to Triple-A Pawtucket and reassigned outfielder Darnell McDonald to Minor League camp. There are 37 players left in Major League camp.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.