The veteran midfielder was occupying the team's third Designated Player spot, but the Sounders are now free to add a new DP. Grenadian
Joseph, 35, is a four-time MLS Best XI player and eight-time All-Star, but injuries and form have limited him to just seven games with the Sounders this season.
Seattle originally acquired Joseph in a trade with Chivas USA prior to the season. The Grenadian is best known for his 10-season stint with the New England Revolution.
With the move, the Sounders now have one free Designated Player spot, which the club is expected to use before the summer transfer window closes on Aug. 9.