SAN DIEGO -- Astros left fielder J.D. Martinez, who didn't play in Monday's series opener, was back in the lineup Tuesday, but not before putting in some extra hitting work prior to the game.

Martinez took early batting practice and hit off a tee with hitting coach Mike Barnett watching closely.

Martinez had hits in his first two at-bats Tuesday, dropping a bloop double to right field in the first inning before rapping an RBI single in the third for his team-leading 50th RBI of the year.

"I just feel like I've been missing a lot of pitches lately because I've been under them so I wanted to work off the tee," he said. "I wasn't getting on top and that was the biggest thing. I'm getting in good counts and being patient and not swinging at many balls. I'm swinging at my pitches and fouling them off and that's what's killing me right now."

It's been nearly a year since Martinez made his Major League debut -- he was called up from Double-A on July 30, 2011, after the Astros traded Hunter Pence -- and he's come to understand he constantly has to make adjustments.

"It's kind of surprising a little bit," he said. "It's definitely adjustments on a regular basis. That's part of it, though. I feel like they're adjustments I can make. I haven't felt overmatched. For the most part, I feel like when I miss balls, I foul them off and that's not usually me."

Lowrie out four to six weeks with leg injury

SD@HOU: Lowrie ties game early with a solo home run

SAN DIEGO -- Astros shortstop Jed Lowrie was relieved to learn Tuesday he wouldn't need to undergo surgery, though the diagnosis wasn't all good news. Lowrie was examined by a team doctor in Houston and told he will be out four to six weeks with a nerve injury in his right leg.

Lowrie suffered the injury trying to make a force at second base Saturday night in San Francisco. He was stretching to take a throw from second baseman Jose Altuve when Giants outfielder Gregor Blanco slid into Lowrie's leg. Blanco's foot pinned Lowrie's leg, and he sprained his ankle and twisted his knee.

"When I look at the replay and the way my ankle and knee were contorted, the fact I didn't need surgery is a good thing," he said. "We'll just take it day by day."

Lowrie's injury is to the peroneal nerve, which is a branch of the sciatic nerve and supplies movement and sensation to the lower leg. He will be on crutches and wear a protective brace for at least a week before beginning rehab exercises.

"From what the doctor told me, I'll just have to wear the brace for a week or two and let the ligament tighten back up and then start the rehab process," Lowrie said.

Lowrie was hitting .253 with a club-best 14 homers and 36 RBIs through 80 games and losing him is a big blow to an Astros team that had batted just .201 in the previous 15 games before Tuesday.

Marwin Gonzalez, a Rule 5 pick last December, will get most of the playing time at shortstop for now, but general manager Jeff Luhnow hasn't ruled out going outside the organization for shortstop help.

"He's done a good job and we're certainly thankful we have him, but we need to consider other options, whether it's bringing up somebody else from Triple-A or looking outside the organization," Luhnow said. "At this point it could go faster, but four to six [weeks] is what we're expecting."

Lowrie has spent time on the disabled list in each of the last four seasons, including this year, when he injured his thumb in Spring Training. He hurt his shoulder last year in a collision with Red Sox teammate Carl Crawford and missed time in 2010 with mononucleosis.

"I've never approached it as being frustrating," he said. "There's nothing I could have done any differently. I was playing the game and trying to get an out. I just have to get it better and get back out there."

Meanwhile, catcher Jason Castro, who was also played on the disabled list on Sunday (retroactive to July 8), had his ailing right knee examined Tuesday in Houston. Astros manager Brad Mills said Castro's knee is still swollen and shows some meniscus damage, but Luhnow remains hopeful Castro can return with a week.

"We're still hopeful it's going to be a short-term situation and he can come back when he's eligible and shortly after," Luhnow said. "We'll know more in the next couple of days."

Castro, who missed all of last season after undergoing ACL surgery on his right knee, was hitting .254 with two homers and 20 RBIs in 20 games this year but had begun to experience some soreness in the knee.

"He's going to have more fluid drained [Wednesday] and hopefully we'll know more in three or four days," Luhnow said.

Astros get inside look at Navy SEAL facility

SAN DIEGO -- For the third year in a row, the Astros were invited to visit the Navy SEAL training facility, located at the Naval Special Warfare Training Center in Coronado, Calif.

Astros general manager Jeff Luhnow and players Justin Maxwell, J.D. Martinez, Brett Myers, Chris Snyder, Lucas Harrell, Brian Bixler and Brian Bogusevic were among those who got a behind-the-scenes tour of the facility Tuesday morning.

Maxwell, who comes from a military family in which both his father and mother were officers in the Navy, always has been intrigued by the Navy SEALs.

"I had seen a few documentaries on the Discovery Channel and National Geographic, so it was actually cool to be out there to see the guys," Maxwell said. "It was cool to be able to get a glimpse of what these guys have to do to become the elite fighting force of the United States."

The highlight was being able to fire live weapons, including an M4 rifle and a handgun, in an indoor range. Maxwell and Bixler were among those who had never fired a weapon before, and Luhnow said Myers, Snyder and Harrell handled their weapons like pros.

"It was cool, man," Bixler said. "They were really nice and informative and really open to allowing us to be in their personal space and see what they do every day. We were able to see how they train, where they train and watch some guys in action. We got to shoot some guns, too, which was very cool."