WASHINGTON -- One day after straining his left hamstring while swinging the bat, outfielder Travis Buck wasn't available to play Thursday against the Nats.

Buck, who started the previous three games, could return by the weekend and doesn't appear to be headed to the disabled list.

"It's still really tight," Buck said. "My hamstring is the anchor of my swing. I can still feel it a little bit in my stance. It's obviously not as bad as I've had in the past, so hopefully I'll get a lot of treatment on it and be back out sooner rather than later.

"It depends on how I feel and if I'll be able to produce, than just throw me out there. I don't want to hurt the team if I feel like I won't be able to have a good swing."

Buck hit in the cage underneath the stands on Thursday, but didn't take batting practice on the field.

"I was kind of feeling for it a little bit and kind of guiding it in the cage," Buck said. "Hopefully they won't need me and they'll score some runs. I want to be out there every day, but you realize what the smart thing is to do."

Buck said the knot in his hamstring is higher on his leg, which is a good sign.

"It's going to take a little bit of work to get in there, but it's more playable than if it was lower in the hamstring," he said.

Downs in starting lineup for first time

WASHINGTON -- It took nearly two weeks into the regular season, but infielder Matt Downs finally was in the starting lineup for the first time on Thursday. Downs, who has been limited to six pinch-hit at-bats, started at third base and hit sixth in the lineup. He had one of three triples for the Astros in the first inning against the Nationls.

"It's kind of hard to get your timing early in the year with an at-bat here and at-bat there, and it's always good to get three or four at-bats in one night to try to get some timing and track some pitches and see some balls," Downs said. "[Manager Brad Mills] was good about getting me in there last year."

Downs, who led the Majors with 15 pinch-hit RBIs last year and averaged a homer every 19.9 at-bats, was hitless in his first six at-bats entering Tuesday.

Mills wanted to give him a start earlier in the year, but third baseman Chris Johnson and second baseman Jose Altuve - the starters at the two positions which Downs is most likely to play on defense - were off to hot starts at the plate.

"It's hard to pull a guy out of there if he's hitting real well, and I don't blame him," Downs said.

Mills said Downs will likely get at least one start this weekend against either Dodgers left-handers Ted Lilly (Friday) and Clayton Kershaw (Saturday).

Three Astros on first All-Star ballot

WASHINGTON -- Online balloting to vote for players for the 2012 Major League Baseball All-Star Game gets underway Friday, with Astros outfielder J.D. Martinez, second baseman Jose Altuve and outfielder Brian Bogusevic making their first appearance on the ballot.

The online ballot is up through June 28 -- exclusively at MLB.com and all 30 club sites, wired or mobile -- allowing you to cast up to 25 ballots per email address.

"That's pretty cool," Martinez said. "I know it's a long shot, but it's pretty neat. It's obviously one of my goals and it's tough to do. You just have to keep working hard and have a good season and just do all the things right and you never know. You could put yourself in a good spot."

Also on the ballot for the Astros is first baseman Carlos Lee, catcher Jason Castro, shortstop Jed Lowrie, third baseman Chris Johnson and outfielder Jordan Schafer.

The 2012 AL and NL All-Star Teams will be unveiled on Sunday, July 1, on the 2012 MLB All-Star Game Selection Show presented by Taco Bell, televised nationally on TBS. The NL All-Star Team will have eight fan-elected starters.

The All-Star Game is slated for July 10 at Kauffmann Stadium in Kansas City.

Worth noting

• Astros manager Brad Mills and shortstop Jed Lowrie were both invited to be part of Friday's 100th anniversary celebration at Fenway Park. Of course, Mills, the former Red Sox bench coach, and Lowrie, who played four seasons in Boston, will be busy. The Astros play the Dodgers on Friday in Houston.

• Astros catcher Jason Castro will be signing autographs for free from 1-2 p.m. Saturday at the team store inside Union Station.