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6/9/2014 11:19 P.M. ET

Walker takes step forward in Triple-A rehab start

Top Mariners pitching prospect Taijuan Walker gave up two home runs Monday at Memphis in his third Minor League rehab stint since returning from a layoff to rest his sore right shoulder, but his five-inning stint for Triple-A Tacoma was his longest since being shut down in mid-April.

The hard-throwing 21-year-old allowed four hits and two runs with one walk and three strikeouts in a 77-pitch outing.

Despite a leadoff home run in the first inning to Memphis second baseman Greg Garcia and a two-out shot in the second by left fielder Thomas Pham, Walker took a big step forward from his previous outing when he lasted just two innings and had command issues while walking four and hitting a batter in a 56-pitch outing at El Paso.

Walker, ranked the Mariners No. 1 prospect and No. 6 in baseball by MLB.com, had expected to contend for a spot in Seattle's rotation this year. But he experienced shoulder soreness at the outset of Spring Training, then was shut down for a second-time after making rehab starts at Class A Clinton and Double-A Jackson in early April.

The Mariners have struggled to fill the fifth spot in their rotation and Walker is lined up to pitch again this Saturday, which coincides with the next start for fifth-starter Erasmo Ramirez at Safeco Field against the Rangers.

The Mariners are currently listing Ramirez at the probable starter in that game and he threw 4 2/3 scoreless innings Monday at Tampa Bay, though he walked five and gave up four hits while continually working in and out of trouble.

It remains to be seen if the club feels Walker needs at least one more rehab start to build up his innings and pitch count before he'll be considered. The 2009 first-round supplemental Draft pick started three games for Seattle at the end of last season, going 1-0 with a 3.60 ERA after putting up a combined 9-10 record and 2.93 ERA in 25 starts with Tacoma and 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.