<img height="1" width="1" style="display:none" src="http://b.scorecardresearch.com/p?c1=2&amp;c2=15743189&amp;cv=2.0&amp;cj=1&amp;&amp;c5=&amp;c15=">

Nationals trade Michael Morse to Mariners

Rob Carr/Getty Images Michael Morse, who hit 64 home runs for the Nationals over the last three seasons, was traded to the Mariners on Wednesday.

Cole returns as part of three-team deal

The Nationals traded outfielder Michael Morse to the Seattle Mariners for pitching prospects A.J. Cole, Blake Treinen and a player to be named Wednesday.

Morse, 30, was made expendable when Washington acquired center fielder Denard Span in a trade with the Minnesota Twins last month and then re-signed veteran first baseman Adam LaRoche last week. With Bryce Harper and Jayson Werth expected to start at the corner outfield positions, there was nowhere left for the big slugger to play.

Instead, the Nats traded him in a three-way deal that included the Oakland A's. Washington welcomes back Cole, 21, a right-hander who was one of four prospects sent to Oakland in the Gio Gonzalez trade last winter. Cole was one of the Nats' prized pitching prospects at the time but struggled at high-A Stockton in the California League (0-7, 7.82 ERA) and was demoted to low-A Burlington. There he stabilized with a 6-3 record in 19 starts and a 2.07 ERA. Cole, a 6-foot-4, 180-pounder, was still ranked by Baseball America this month as Oakland's third-best prospect. He was originally drafted by Washington in the fourth round of the 2010 draft.

Over the last three years, only 17 big leaguers with at least 1,000 at-bats have a better slugging percentage than Morse (.516). He returns to Seattle, the team Washington acquired him from after spring training in 2009. Knee and shoulder injuries derailed Morse's career with the Mariners. Seattle sent catcher John Jaso to the Athletics in the trade.

Oakland will provide the player-to-be-named-later to the Nats. It is the third trade involving Washington and Oakland in 13 months. Cole went to the Athletics along with pitchers Brad Peacock and Tom Milone and catcher Derek Norris for Gonzalez, who was a finalist for the National League Cy Young Award in 2012. In August, the Nats acquired catcher Kurt Suzuki from Oakland for catching prospect David Freitas.

Treinen, 24, was 7-7 with a 4.37 ERA for Stockton last season. He started 15 games and was a reliever in another nine. Treinen is a 6-foot-4, 215-pound right-hander who was drafted by Oakland in the seventh round of the 2011 draft. He played college baseball at South Dakota State.