Gary Cameron/Reuters

The Nationals, according to multiple people with knowledge of the situation, sent slugger Michael Morse to the Seattle Mariners in a three-way trade that netted them right-hander A.J. Cole, the pitching prospect who last winter served as the centerpiece of the trade that brought Gio Gonzalez from the Oakland A’s to the Nationals. The Mariners sent catcher John Jaso to the Oakland to complete the deal.

The Nationals also received 24-year-old minor leaguer Blake Treinen and a player to be named later from the A’s, the team announced.

The Nationals had been in discussions to trade Morse for weeks, talks that intensified last week once the Nationals re-signed first baseman Adam LaRoche. With LaRoche in the fold and their outfield full, Morse became expendable. Morse, who turns 31 in March, has one year remaining on his contract and will make $6.75 million this season.

Cole, a 6-foot-4 right-hander who can hit the high-90s with his fastball, went 0-7 with a 7.82 ERA when Oakland started him at high-A Stockton. After a demotion to low-A, Cole improved and went 6-3 with a 2.07 ERA the rest of the season. His star has dimmed since the Nationals dealt him to Oakland, but he is still only 21 and still has the potential Washington saw in him when they signed him to a $2 million bonus out of the 2010 draft.

Cole’s re-arrival will help replenish the pitching in the Nationals’ minor league system, which had been depleted by several recent trades. They shipped top pitching prospect Alex Meyer to the Minnesota Twins for center fielder Denard Span, and they also dealt Brad Peacock and Tommy Milone to Oakland in the Gonzalez trade.

While Cole returns to Washington, Morse will return to the team the Nationals acquired him from in 2009, when Morse was still a Class AAA hitter without a position. During his time in Washington, Morse became a fan favorite – fans at Nationals Park sang his walk-up song “Take On Me” when he came to bat – a pivotal part of a lineup that turned a downtrodden franchise to a World Series contender. Over the past three seasons, Morse compiled an .861 OPS and 64 home runs, including 31 in his breakout 2011 season.

General Manager Mike Rizzo can only hope his second trade involving Morse works as well as his first. In late June 2009, Rizzo, then the Nationals’ acting GM, engaged in trade discussions with the Mariners. They needed a speedy outfielder who played strong defense. Rizzo had his eye on a Class AAA prospect who had outgrown shortstop, who had no defined position, some durability questions and a bat that wouldn’t quit.

After Mariners GM Jack Zduriencik relented, he finally agreed to deal Morse for Ryan Langerhans. Rizzo thought he had traded for a player who might become the right-handed half of an outfield platoon. Morse instead blossomed into a star slugger. Langerhans appeared in 21 games over the 2011 and 2012 seasons combined.

Treinen, 24, went 7-7 with a 4.37 ERA and 4.0 strikeout-to-walk ratio in 24 games, including 15 starts, at Class High-A Stockton last year. Stockton, where Cole struggled, is part of the California League, which annually produces inflated offensive numbers.

With Morse traded, the Nationals will rely on Tyler Moore as their primary right-handed pinch-hitter off the bench and as a backup to LaRoche at first base.