In three rehabilitation starts with Triple-A Buffalo, Happ went 0-2 with a 6.75 ERA in 13 1/3 innings. The lefty had gone 2-2 with a 4.91 ERA in seven starts with Toronto before going on the disabled list.
Happ wasn't expected to land in Seattle until just before the scheduled 7:10 p.m. PT first pitch and it is unknown if he'll wait to join the team until Tuesday.
Schwimer was a 14th-round selection by the Philadelphia Phillies in the 2008 Draft and had appeared in six game for Buffalo this season.
Josh Liebeskind is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.