The club also announced that right-handed reliever Stephen Pryor is going on the seven-day Minor League disabled list after being diagnosed with an impingement in his throwing shoulder while pitching for Tacoma.
Paxton was examined by team physician Ed Khalfayan after lingering soreness from his three-inning outing, when he threw 62 pitches in his first game action since straining his left lat muscle on April 8.
The 25-year-old will be prevented from throwing until the inflammation goes away, which the club hopes will be within a week or so. There had been hope that Paxton was close to rejoining the rotatation after another rehab start, but that now will be on hold.
"It is tough when you get that close to being back and then have something hold you back a little bit, but we just have to take it one day at a time right now and take care of it so I can get back out there as soon as possible," said Paxton. "I think it's just from upping the intensity after not throwing for a while. I don't think it's a big, big deal. I just have to give it a little time."
Paxton opened the season with the Mariners and was 2-0 with a 2.25 ERA before being shut down following his second start of the year in Anaheim. The British Columbia native is 5-0 with a 1.75 ERA in six career starts after debuting last September.
Walker, 21, will throw about 70-75 pitches in his first outing for the Rainiers, manager Lloyd McClendon said. The hard-throwing youngster is ranked as the No. 6 prospect in baseball by MLB.com and was 1-0 with a 3.60 ERA in three late-season starts for Seattle last year.
The 2010 first-round supplemental Draft pick was expected to open this season in the Mariners rotation, but reported to Spring Training with a sore shoulder that limited his early work and then had to be shut down again in mid-April after making two Minor League rehab starts with Class A Advanced High Desert and Double-A Jackson.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.