Walker outpitched Salt River's Archie Bradley, the seventh overall pick in the first round of the 2011 Draft, as the 22-year-old Diamondbacks right-hander gave up five hits and three runs in three innings.
Walker, also 22, was the 43rd overall pick in 2010 as a compensation-round selection by the Mariners. He is expected to start six games and pitch about 25-30 innings in the AFL as the Mariners look to get him some extra work after he missed the first two months of the season with shoulder issues.
Walker has a 2.00 ERA in two AFL starts (two earned runs in nine innings) after going 2-3 with a 2.61 ERA in eight appearances for Seattle this past season and 7-4 with a 4.37 ERA in 16 Minor League outings.
Just as in his first AFL game, Walker wound up with a no-decision after turning a lead over to his bullpen. This time Surprise coughed up a 4-1 advantage as Salt River scored six times in the top of the seventh off Padres prospect Tayron Guerrero.
Surprise is now 3-3 after the first week of the 32-game season. Mariners position prospects D.J. Peterson, Patrick Kivlehan and John Hicks all played in Monday's outing as well. Peterson was 0-for-4 and is now batting .231 as the team's primary third baseman. Kivlehan played first base again and went 1-for-4 with a walk to put his average at .238, while Hicks was 0-for-3 with a walk and run scored as the young catcher is hitting .273.
Matt Brazis, one of three Mariners relievers on the staff, replaced Guerrero with two outs in the seventh and gave up a hit and Guerrero's final run before ending the six-run uprising with a strikeout. Brazis then retired the side in order in the eighth with two more strikeouts.
Center fielder Rusney Castillo, who signed a seven-year, $72 million deal with the Red Sox out of Cuba in August, went 2-for-5 with an RBI double and leads Surprise with a .350 batting average in his first four games.
Each team in the AFL is made up of top prospects from five Major League franchises.
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.