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6/30/2014 10:30 P.M. ET

Mariners recall Walker to start against Astros

HOUSTON -- As expected, the Mariners recalled top right-handed pitching prospect Taijuan Walker on Monday afternoon in time to start the evening's game against the Astros at Minute Maid Park.

Walker's spot on the 25-man roster was cleared when rookie outfielder Stefen Romero was optioned to Triple-A Tacoma following Sunday's game against the Indians.

Walker, 21, made his season debut with Seattle in the same stadium in which he made his first Major League start. In that game, last Aug. 30, Walker picked up the victory while allowing just one earned run in five innings against Houston.

Walker pitched against the Astros twice in his three starts with the Mariners late last season, going 1-0 with a 3.60 ERA.

The hard-throwing youngster was expected to open this year in the Mariners' rotation, but Walker missed most of the spring with a sore right shoulder and spent time early this season on the disabled list before working his way through a pair of Minor League rehab assignments and then a stint with Tacoma.

Walker, the No. 5 overall prospect in baseball according to MLB.com, went 2-1 with a 4.11 ERA in six starts with the Rainiers and was named the Pacific Coast League Pitcher of the Week on Monday for his four-hit shutout performance in his Tacoma finale against Oklahoma City last Tuesday.

Romero, 25, appeared in 51 games and hit .196 with 18 runs, six doubles, two triples, three home runs and 11 RBIs. Romero hit .263 (15-for-57) on the road and .154 (14-for-91) at Safeco Field. McClendon said the youngster would benefit from playing every day again in Tacoma.

"It gives him a chance to go down and get some at-bats," McClendon said. "I think this young man has a bright future, but I know -- having lived it and having done it -- it's the hardest job in baseball to play once or twice a week and expect to be productive. If he gets back here, he'll be a better player for us as a result."

The Mariners chose to keep right-handed reliever Brandon Maurer and go with an eight-man bullpen for now, which leaves just three position players on the bench each night. But McClendon said Maurer's impressive performance -- three scoreless innings with six strikeouts in two appearances - warranted the move.

"There are a couple factors that played into it," McClendon said. "No. 1, our pitching has been our foundation, and this gives us a chance to protect it a little and shorten up ballgames. We all know offensively we're challenged a little, so it gives us a chance to shorten games when we can and protect guys and not overuse anybody. And this guy was pretty impressive coming out of the bullpen. It's very hard to ignore what he did two times coming out of the bullpen, so we'd like to see more of him."

Even without Romero, the Mariners have five outfielders with Dustin Ackley, James Jones, Michael Saunders, Endy Chavez and Cole Gillespie, while Willie Bloomquist can also play there if needed.

"Bloomy can play all over the place, and that obviously gives you a little more flexibility," McClendon said. "Listen, it's not like we've had this tremendous bench that we've run through every game. That factored in as well. In the American League, you have a DH. You don't pinch-hit that much and use your bench that much. And we don't play the NL until Aug. 18, so it gives us a chance to give us a little more run with our pitching."

Timeline materializes as Paxton throws 'pen

HOUSTON -- Mariners rookie southpaw James Paxton threw a 35-pitch bullpen session at Minute Maid Park prior to Monday night's game with the Astros as he continued his recovery from shoulder issues that had sidelined him since his second start of the season.

Paxton is accompanying the team on its current trip and said he would throw another bullpen of 45 to 50 pitches on Friday in Chicago and could then be ready for a simulated game when the club returns to Seattle next week.

"Everything is going forward as planned, so far," the 25-year-old said. "It felt really good today. The ball was coming out well. Everything felt smooth."

Paxton has gone 5-0 with a 1.75 ERA in six Major League starts going back to last season, but he has been unable to stay healthy this year after straining the lat muscle behind his left shoulder in his second outing of the year. He worked his way back through a similar rehab program but had to be shut down again after new soreness cropped up in his shoulder after a three-inning start for Triple-A Tacoma on May 24.

So for now, he is just happy to be back traveling with the team and working out instead of being stuck behind in the Safeco Field training room.

"It feels a little better, more like you're part of the team," he said. "I'll be out there tonight cheering the guys on in the dugout and watching my buddy [Taijuan Walker] coming back."

Walker, who spent considerable time in the training room with Paxton this spring, made his Mariners season debut Monday night against the Astros.

Worth noting

• Designated hitter Corey Hart and first baseman Justin Smoak remained with Triple-A Tacoma on rehab assignments as the Rainiers returned home to open a series with Reno on Monday. Smoak hit .216 (8-for-37) in 10 rehab starts while coming back from a strained quad, while Hart was hitting .241 (7-for-29) in seven games.

"They're doing fine. They're getting better," McClendon said. "But I would say it'll probably be a few more days, for sure."

Position players can spend up to 20 days on a rehab assignment, which means Smoak has until July 8 and Hart has until July 11 before they have to rejoin the Mariners or be optioned to the Minors.

• The Mariners were hitting .266 with runners in scoring position entering Monday, which was fourth in the American League and the club's best prior to the All-Star break since hitting .290 in 2007.

• With Sunday's save against the Indians, Fernando Rodney moved into a tie for the American League lead with the Royals' Greg Holland with 23. Rodney had nine saves in May and June before Monday, joining J.J. Putz (May-June, 2007) as the only Mariners closers to record nine saves in back-to-back months. The Mariners have only had nine times in which any reliever has totaled nine saves in a calendar month.

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.