The roster move made to get third baseman Jack Hannahan on the team came as a surprise to pitcher Brandon Morrow - because it was Morrow sent out.
After six starts, Morrow will stay with the Mariners through the All-Star break, then step into the Rainiers starting rotation.
There, he'll join Ryan Rowland-Smith and Jason Vargas, who have also started in Seattle and Tacoma this year.
"We don't need a fifth starter after the break until July 25," manager Don Wakamatsu said. "We fan slide Garrett Olson into the rotation after the break until we make any other decision.
"We're sending Brandon out with specific goals, to improve his secondary pitches and continue to work on pitch efficiency. As I told him a few minutes ago, he's a big part of our close future. We stretched him out up here, he worked with (pitching coach) Rick Adair. Now he's got a definitive plan to work on."
In his six starts, Morrow was 0-1 and went beyond five innings only one time.
Hannahan is expected at Safeco Field momentarily - he's flying in from Sacramento - and is in the starting lineup tonight.