© 2013 MLB Advanced Media, L.P. All rights reserved.

04/21/2013 7:29 PM ET

Martinez homers in return from oblique injury

HOUSTON -- It has been a frustrating start to the season for outfielder Fernando Martinez, who was set to make the Opening Day roster before an oblique injury sent him to the disabled list. He rejoined the club on Sunday and started in left field in his 2013 debut.

His return to Minute Mate Park, though, was more enjoyable. Martinez smacked the first pitch he saw in 2013 to the left-field seats for a two-run home run.

Martinez was 2-for-18 on a Minor League rehab assignment at Triple-A Oklahoma City and was activated when J.D. Martinez went on the disabled list with a right knee sprain. The often-injured outfielder said he is 100 percent, but wants to prove he's healthy.

"It's very important for me because over my career there have been a lot of injuries," he said. "I have to prove it to the people, prove it to myself I can play every day."

Martinez, a former top prospect of the Mets who was claimed off waivers by the Astros on Jan. 11, 2012, hit .344 in the spring with one homer and eight RBIs, and he was set to start on Opening Day before straining his oblique.

"That was a little hard for me because I worked hard in the offseason to be here, to make the Opening Day roster," he said. "That's part of the game, and it's just something that happens. You have to be ready for the next step and the opportunity that's here now."

Oberholtzer makes Major League debut

HOUSTON -- Nothing like catching a 5 a.m. flight when you haven't slept, arriving at the ballpark at 11 a.m. and making your Major League debut only a few hours later. For Brett Oberholtzer, it was nothing short of a dream come true.

The Astros called up the lefty from Triple-A Oklahoma City, informing him late Saturday night in Albuquerque, N.M. he was headed to Houston. He pitched two innings in relief Sunday and gave up a pair of solo home runs and a double in Houston's loss to Cleveland.

"I was a little nervous, not as nervous as I thought I would be," said Oberholtzer, who was acquired from the Braves in the 2011 trade for Michael Bourn. "It's all about how to handle it and continue to make progress."

Oklahoma City manager Tony DeFrancesco summoned Oberholtzer into his office following Saturday night's game and scribbled on paper for a few minutes before telling him nonchalantly he was going to Houston. Once he gathered his thoughts, he called his parents in Delaware.

"It meant the world to me," he said. "Growing up as a kid you always want to become a professional baseball player. I had that goal since I was 5 in tee ball. I need to continue to work hard and make strides and I'm glad I started my career."

Oberholtzer had been starting for the RedHawks, but he'll pitch in long relief for the Astros. Paul Clemens was unavailable Sunday after throwing 3 1/3 scoreless innings Saturday, and Dallas Keuchel threw 2 2/3 innings before being sent down.

"Having guys down in the bullpen as long guys is vital to the overall success of the team," Astros manager Bo Porter said. "You look at the short sample in which we've had this year, and they've saved us quite a few times, times when our starter has gotten in trouble early and other times we wanted to bridge to the back end of our bullpen."

Oberholtzer made three starts for Oklahoma City, going 0-2 with a 9.49 ERA. In 12 1/3 innings pitched, he allowed 18 hits, 13 earned runs and four walks.

Carter gets day off from lineup amid slump

HOUSTON -- Left fielder/designated hitter Chris Carter was out of the starting lineup Sunday for the first time this season, but manager Bo Porter said it had nothing to do with his 0-for-13 slump that had dropped his batting average to .219.

With Fernando Martinez being activated from the disabled list, Porter started Martinez, a left-handed bat, in left field against Indians right-hander Ubaldo Jimenez. Even so, Carter said the timing of the day off was good.

"It's a good day to relax and clear your mind," he said.

After a six-game stretch in which he batted .462 (12-for-26) with four homers and seven RBIs and was as locked in as you can get, Carter has gone 1-for-19 with no homers and two RBIs in the six games since, entering Sunday.

"You look at the last couple of games and he's starting to expand the zone a little bit more," Porter said. "We had that conversation and I told him, 'You have to get back to getting pitches you know you can handle and not expand your strike zone.'"

Porter a firm believer in benefits of infield shifts

HOUSTON -- The Astros have been routinely employing defensive shifts on the infield this year, which involves moving a middle infielder to the opposite side of the base. Against left-handed hitters, second baseman Jose Altuve plays in shallow right field and the shortstop plays on the first base side of second base.

It's a product of the data compiled by the Astros' analytics department, but manager Bo Porter said the pitcher-batter match-ups play into defensive positioning more than anything else.

"In Washington [where Porter was the third-base coach], we employed the same system because I'm a firm believer you have to defend the portion of the field that the guy has the greatest probability of hitting the ball well," Porter said.

Porter believes the Astros have been more successful than not when using the shift on the infield this year. It hasn't always worked -- Josh Hamilton rolled a grounder to Altuve in shallow right field and wound up with a hit -- but he's a firm believer in the practice.

"I know when you look at the spray charts and the pitcher we have on the mound and percentages of pitches they throw to certain zones, you have a probability of where the ball is going to be hit well, and I keep stressing the well part," he said.

Porter has spoken at length with sluggers David Ortiz and Adam LaRoche, both of whom see frequent defensive shifts, about what goes through their minds at the plate. Do they think about going the other way to take advantage of the open spaces?

"If they did that, it's still advantage us," Porter said. "Because you're talking about a guy who has the ability to change the game with one swing and you're making him do something he doesn't want to do, and it's mission accomplished."

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.