Outfielder Kevin Thompson was optioned to Triple-A Indianapolis, right-hander Jonah Bayliss was designated for assignment, and pitchers Elmer Dessens and Casey Fossum were reassigned to Minor League camp.
The designation of Bayliss opened a spot for infielder Luis Rivas, who was added to the 40-man roster.
General manager Neal Huntington and manager John Russell's decision to start Thompson at Triple-A was a prelude for an announcement to come later in the day.
Though Huntington was noncommittal that both Nyjer Morgan and Nate McLouth would both be on the Opening Day roster, immediately following Tuesday's game vs. the Twins, both players were told that they had earned a spot on the Pirates' 25-man roster.
McLouth was then given the Opening Day nod.
Coming into Tuesday's game, McLouth seemed the most likely choice to win the job after a solid spring in which he's batted .333 with nine extra-base hits and eight RBIs. Morgan entered Tuesday hitting just .232.
With McLouth and Morgan both on the team, that now leaves four spots open on the bench.
Rivas, catcher Ryan Doumit and utilitymen Chris Gomez and Doug Mientkiewicz are the likely candidates to fill the vacant spots.
As for Bayliss, who made six appearances this spring, the Pirates have 10 days to either trade him, release him or outright him to Triple-A, assuming he clears waivers first.
Huntington and the Pirates hope that Bayliss will clear waivers and remain with the organization.
"We still think Jonah has a Major League future," Huntington said. "We still have some tough decisions to make in the bullpen. But as we sit here right now, Jonah was not in that final mix of players."
Neither was Dessens, who was sidelined with lower back stiffness early in Spring Training, hurting his chances of being ready for Opening Day. The 11-year veteran gave up only two hits in three appearances this spring.
Fossum will join Dessens at Minor League camp after making five appearances in the team's Grapefruit League games, giving up five earned runs on seven hits in four innings of work.
There are now 33 players in camp -- 17 pitchers, two catchers, nine infielders and five outfielders.
Todd Krise is an associate reporter for MLB.com. MLB.com reporter Jenifer Langosch contributed to this report. This story was not subject to the approval of Major League Baseball or its clubs.