HOUSTON -- Catcher Welington Castillo was placed on the 15-day disabled list Tuesday, retroactive to Saturday, with a sprained MCL in his right knee, which opened a roster spot for lefty Travis Wood, who started Tuesday against the Astros.

Castillo, 25, was batting .185 with two doubles, one home run and four RBIs in seven games. He injured his knee last Friday at Wrigley Field on the last pitch Kerry Wood threw in the eighth inning.

Castillo is the third catcher to go on the disabled list. Steve Clevenger, sidelined since April 26 with a strained right oblique, is currently rehabbing at Triple-A Iowa. He was the designated hitter on Tuesday and went 1-for-2 with a single. He could be ready when the Cubs return home Monday.

Geovany Soto, the Cubs' No. 1 catcher, was placed on the DL on Saturday after needing arthroscopic surgery on his left knee. He will miss three to four weeks.

Wood, 25, made his second start for the Cubs on Tuesday, taking Chris Volstad's spot in the rotation. In seven starts with Iowa, Wood was 3-3 with a 4.57 ERA.

LaHair out of starting lineup, but enters in sixth

HOUSTON -- Bryan LaHair got most of Tuesday off.

The first baseman, who is 1-for-20 in his last seven games, did not start against the Astros to get what Cubs manager Dale Sveum called a "mental breather." LaHair has started 36 of 43 games, but he did enter as a defensive sub in the sixth.

"He's never really gone through this mental grind in the big leagues and played this many games in a row," Sveum said.

LaHair, batting .315 overall, took early batting practice and also was able to work out in the weight room and on the field before the game. Can he explain the last few games?

"I've had some rough luck for a few games there where I had no results," said LaHair, whose 10 home runs lead all regular first basemen in the Majors. "I thought I was having good at-bats. A couple games against the White Sox, I just didn't feel good at the plate. Last night, I gave the first at-bat away and I was [ticked] about that, and then I just missed a three-run homer [in the third]. It is what it is, it's baseball. Everybody goes through it. I go through it a couple times a year. I feel like I'm one click away from getting back to where I was."

He felt as if he knew what to expect but didn't react. Take Monday against the Astros' Bud Norris.

"I could've bet my life the second pitch would be a curveball, and for whatever reason, I just didn't swing and I knew it would be there," LaHair said. "Just like my second at-bat -- I knew the first pitch would be a fastball and I just missed it."

The early batting-practice session was loud, as players hit more than a few balls into the empty outfield seats.

"It's always nice to take early BP in parks like this," Sveum said. "Sometimes that's what it takes."

Even Tony Campana, more likely to get an inside-the-park home run than launch one over the fence, was able to drop a ball into the seats.

"Like I said, it's nice to take early BP here," Sveum said, smiling.

Hill working hard to learn new pitchers

HOUSTON -- Koyie Hill knows most of the Cubs' pitchers, so the transition from catching young Double-A pitchers to the big leagues has been relatively easy. It also helps that he's the father of two young girls.

Hill signed Saturday with the Cubs, who were in need of help after three catchers were sidelined with injuries. He was with the Cubs from 2007-11, so he knows most of the pitching staff and is learning quickly about the others.

"I had fun catching Paul [Maholm] the other day," Hill said. "He was somebody I always watched, because I never got to face lefties that much. I enjoyed how he competed. Win, lose or draw, you've got a guy who can compete and is capable of getting something out of his ability. Then you're going to have some success and there's something to go on."

The process now is to learn more about the pitchers as people, not just what they throw.

"I wouldn't say it comes easy, but it's something I've learned over the years on how to trim the fat on that stuff and get to the core of a guy pretty fast, just by hanging around them and being in different situations," Hill said. "A lot of times, it's just listening, and that's something I learned from my girls. I've often compared my girls to the bullpen and the pitching staff, because you have to figure out what makes each one of them work. No situation is like the other."

Extra bases

• Chris Volstad got a win. The right-hander, sent to Triple-A Iowa after opening the season 0-6 in eight starts, won Tuesday for Iowa. He gave up four runs on seven hits over six innings in the Iowa Cubs' 18-8 victory over Tacoma. Volstad hasn't gotten that kind of run support in his last 18 starts in the big leagues. His teams have averaged 2.66 runs while he's on the mound in that stretch.

Anthony Rizzo had three hits for the Iowa Cubs, including his 15th home run, and now has 42 RBIs for the season.

• Cubs manager Dale Sveum will miss Wednesday's game to attend his son, Rustin's, high school graduation from Desert Mountain High School in Scottsdale, Ariz.

It's an evening ceremony, so Sveum won't be able to watch the Cubs' series finale against the Astros.

"He'll be graduating, so most of that time, I'll be looking at my phone [for updates]," Sveum said.

Bench coach Jamie Quirk will take over on Wednesday.

• Reliever Blake Parker, bothered by a tight right hamstring, was able to throw on flat ground Tuesday but will only be used in emergency, Sveum said. Parker has not pitched since Saturday.