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4/5/2014 4:25 P.M. ET

Top prospect Walker sharp in rehab start

OAKLAND -- Mariners manager Lloyd McClendon said all went well with Taijuan Walker's first Minor League rehab start Friday for Class A High Desert, and the club's top pitching prospect will make at least one more start this coming week.

Walker, 21, allowed four hits and two runs (one earned) while throwing 72 pitches in 4 1/3 innings in a 5-4 victory over Inland Empire. The Mariners' prospect had one walk and seven strikeouts as he returns from a sore shoulder that sidelined him earlier this spring.

"Everything went great," McClendon said. "He was clean. No setbacks. We'll continue to move forward."

McClendon said it hasn't been determined yet where Walker will make his next start, though he said it'll be at a warm-weather site. If all goes well, Walker could potentially return to the Mariners, but the manager wasn't committing to that yet.

"We'll see," McClendon said. "I would say he'll throw 85-90 pitches next, and we'll see how he feels after that one."

All-Star right-hander Hisashi Iwakuma hasn't started throwing off a mound yet, but he is scheduled for another long-toss session on flat ground Sunday. If all goes well, he'll throw his first bullpen outing midweek and begin building up his arm strength after missing all of spring with a sprained tendon in his right middle finger.

McClendon said Iwakuma likely will do his Minor League rehab stint in Triple-A once he's ready, which will likely be toward the end of April.

"By the time he's ready, I would think the weather will probably be pretty decent, for the most part," McClendon said. "We will probably see him in Tacoma."

Mariners, A's agree to postpone doubleheader

OAKLAND -- With Friday night's game at O.co Coliseum deemed unplayable because of poor field conditions, the Mariners and A's will make up that contest later in the season when Seattle returns on one of its remaining visits.

Mariners player representative Charlie Furbush said Saturday that the exact makeup date remains to be determined, but it will most likely be a doubleheader on May 5, the first day Seattle returns to Oakland for a three-game series.

The two teams are also scheduled for a Sept. 1-3 series in Oakland.

Friday night's postponement was the Mariners' first rainout since May 15, 2011 at Cleveland. The last time Seattle was rained out in Oakland was April 10, 1982.

Mariners manager Lloyd McClendon said the mushy infield, created when a heavy rain fell Thursday night in Oakland after the A's grounds crew didn't put a tarp over the infield, was one of the worst field conditions he'd ever seen.

"I've never had a game canceled for that particular reason," McClendon said Saturday. "I almost turned my ankle [walking across the infield]. It was that bad. It was just not safe."

The makeup situation in such a postponement is up to the players union. Furbush said the A's initially wanted to play a doubleheader on Saturday, but they quickly agreed to delay it until a later date after the Mariners pushed for that idea.

"It was kind of interesting talking to everyone, trying to figure out what the best game plan was," Furbush said. "I was just trying to represent everyone as best as I could. Once we realized, given the playing conditions, that playing one [Saturday] was going to be better for us and wait until we come back to play two, that was the general consensus and that's the way it worked out."

Furbush said players' safety was the foremost concern.

"Part of it was just the playing conditions," Furbush said. "Lloyd went out there and almost twisted his ankle. It was pretty bad. You don't want anybody going out there getting hurt. I think the outfield was OK, but the infield, where a lot of hard cuts are being made, that's going to put the safety of everyone in question. So I think it worked out good."

Young's turn in rotation skipped after rainout

OAKLAND -- Chris Young was eager to make his first Major League appearance since 2012 due to shoulder problems, but the veteran right-hander will have to wait longer now after having Friday's Mariners start postponed by poor playing conditions at O.co Coliseum.

The 6-foot-10 Young will be moved into the bullpen as a long reliever for now, as Mariners manager Lloyd McClendon chose to skip the 34-year-old fifth starter and stick with Felix Hernandez on his normal schedule for Saturday. McClendon will keep his other starters in their normal rotation as well for now.

Erasmo Ramirez will start Sunday's series finale against the A's as planned, and James Paxton has Tuesday's home opener against the Angels at Safeco Field. That lines up rookie Roenis Elias for Wednesday's game, with Hernandez coming back Friday for the first contest of a three-game weekend set with the A's.

With two off-days coming up on Monday and Thursday during Seattle's opening homestand, Young may not be in line to return to the rotation until the Mariners head back out on the road to Texas and Miami the following week.

The decision for McClendon wasn't difficult.

"I think anytime you have a chance to keep your No. 1 on schedule, you keep him on schedule," McClendon said. "To me, that just makes more sense. They're No. 1 for a reason."

As for Young, who signed a one-year deal two weeks ago after being released by the Nationals?

"He'll pitch out of the bullpen, and hopefully we'll try to keep him as sharp as we can," McClendon said. "Then we'll reevaluate in a few days and see where we are."

Young, a nine-year Major League veteran, has started 159 big league games in his career, but he has never pitched in relief.

Worth noting

• Nick Franklin went 5-for-8 with a home run and two RBIs as Triple-A Tacoma split a season-opening doubleheader Friday against Albuquerque at Cheney Stadium. Franklin played second base in one game and shortstop in the other.

Blake Beavan gave up one run and four hits with two walks and six strikeouts in seven innings as the Rainiers won the opening game, 11-1. Nick Hill went 1 2/3 innings and took the loss as five pitchers split duties in a 6-3 loss in the nightcap, with Brandon Maurer throwing a scoreless fourth.

• Mariners pitchers set a club record by allowing three runs or fewer in each of the first four games to start the season. The previous record was three, done in 1993, '95 and 2000. Only three MLB teams have opened the year with five games of three or fewer runs since '04.

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.