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09/03/12 6:16 PM ET

Ambriz, Valdez could factor in late-inning equation

PITTSBURGH -- Astros interim manager Tony DeFrancesco said Monday he's still trying to get a feel for the back end of the bullpen. The Astros have shuffled their late-inning bullpen alignment often since relievers Brandon Lyon and Brett Myers were traded in July.

Veteran Wilton Lopez remains the closer, though the Astros haven't had many save opportunities. Newcomers Hector Ambriz and Jose Valdez could be getting some of those eighth- and ninth-inning assignments as the season winds down.

The Astros' bullpen allowed five runs in the eighth inning on Sunday's loss to the Reds and has posted a 5.81 ERA since May 26, which has coincided with the club's greater struggles.

"We're going to continue to work on a plan of execution," DeFrancesco said. "I think Hector has shown he can pitch late in the game, and Valdez is probably going to get an opportunity late in the game. Lopez is still our best bullpen guy. You guys saw him last year when he was good and consistent, and we're going to make sure he has a chance to close games for us."

Valdez was called up Saturday after saving 21 games at Triple-A Oklahoma City, and Ambriz joined the club Aug. 22 after pitching 18 games for the RedHawks. The Astros signed Ambriz as a Minor League free agent on June 21.

Lowrie may begin rehab assignment this week

PITTSBURGH -- The Astros are still hopeful Jed Lowrie will get back on the field at some point before the season is over, and interim manager Tony DeFrancesco said the club could send the shortstop to rehab at Double-A Corpus Christi by the end of the week.

The Hooks ended their season Monday, but Lowrie could join them for the Texas League playoffs before rejoining the Astros. Lowrie was having a solid first season before injuring his right leg July 14 in San Francisco, hitting .253 with 14 homers and 36 RBIs. He hasn't played since.

"We're pleased with Jed," DeFrancesco said. "Jed took batting practice [Sunday] off me and was fielding some ground balls. We're hoping by mid-September we'll see him in some games up here. I know it's real positive with him. I think he's turned the page on his rehab and [head athletic trainer Nate Lucero] is trying to push him a little bit more. That's what will help us down the road."

Marwin Gonzalez, who took over at shortstop for Lowrie, is out with a torn ligament in his left ankle, but could return by season's end. Tyler Greene is now the starting shortstop.

Meanwhile, DeFrancesco said veteran relief pitcher Francisco Cordero had a setback in his rehab. The right-hander has been out since Aug. 2 with a toe injury, but he has also been experiencing some discomfort in his throwing shoulder.

Keuchel, Martinez among four Astros callups

PITTSBURGH -- The Astros called up four more players from Triple-A Oklahoma City following Monday's game -- catcher Carlos Corporan, right-hander Chuckie Fick, left-hander Dallas Keuchel and outfielder J.D. Martinez.

Oklahoma City was eliminated from playoff contention on Sunday night and finished its season with a 1-0 win over Round Rock on Monday. Keuchel got the start, throwing three perfect innings in an abbreviated outing.

"The tough part of all these guys coming up is it's hard for me to get them in games, so we'll be seeing a lot of double-switching, some pinch-hitting earlier in games if we have to," Astros interim manager Tony DeFrancesco said. "There's only eight slots [for position players in the starting lineup] and a ton of players who want that playing time. We're going to prioritize some of our playing time for some of our younger players."

Corporan hit .275 with three homers and nine RBIs in 18 games with the Astros earlier this year, including 15 starts. Martinez, who leads the Astros with 54 RBIs this year, was sent to Oklahoma City on Aug. 9 and hit .233 with no homers in 23 games for the RedHawks.

Fick will also be in his second stint with the Astros, having appeared in nine games earlier this year, posting a 4.85 ERA.

The big league stint will be the third for Keuchel, who has made 12 starts for Houston. In his 16 starts at the Triple-A level this year, Keuchel went 6-4 with a 3.90 ERA.

In addition, the Astros will be adding an extra coach to handle the extra players on the roster. The rules allow clubs to call up one uniformed coach/manager from the Minor Leagues, and the Astros could rotate a couple of those through in September. Minor League medical coordinator Jamey Snodgrass has joined the club to lend a hand to the training staff.

Worth noting

• To make room for Monday starter Edgar Gonzalez on the 40-man roster, the Astros outrighted pitcher Mark Hamburger to Triple-A Oklahoma City after he cleared waivers.

Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.