HOUSTON -- Now that George Springer has joined the Astros can Jonathan Singleton be far behind?

Singleton, a slugging first baseman who was in spring camp with the Astros, went 3-for-4 with a double, triple and home run at Colorado Springs on Wednesday. He's hitting .358 with a .460 on-base percentage with five homers and 18 RBIs in 13 games at Triple-A. Still, Astros general manager Jeff Luhnow said Wednesday his arrival in Houston isn't imminent.

"We're happy that Singleton is off to a good start," he said. "He does not have the Triple-A track record that Springer has yet. We feel he needs to develop that track record. We're always going to be in a position where our fans want our top prospects to come up here sooner than we think they're ready to come up here.

"Last year, it happened to [Jarred] Cosart. We did bring Cosart up at the right time, and when he came up he contributed and helped us win ballgames. He still is showing that deserves to be at this level and is ready to help us win."

Singleton played most of last season at Oklahoma City, hitting .220 with six homers and 31 RBIs in 73 games. Singleton missed the first 50 games of 2013 because of a second failed drug test and admitted this spring he enjoyed smoking marijuana and had spent time in rehab last year. He also said he battled alcohol addiction last year.

Peacock gets opportunity to join rotation

HOU@TEX: Peacock escapes bases-loaded jam

HOUSTON -- With right-hander Lucas Harrell having been designated for assignment, the Astros are going to turn to right-hander Brad Peacock to take his spot in the rotation. He'll make his first start of the season Sunday against the A's -- his former team -- in Oakland.

Peacock, Harrell, Jerome Williams and Dallas Keuchel competed for the final two spots in the starting rotation at the end of spring, with Harrell and Keuchel getting those jobs and Williams and Peacock going to the bullpen. Peacock appeared in three games entering Wednesday, with all three outings lasting at least 2 2/3 innings.

"The plan is to try to get him on the mound at some point [Wednesday] if the game situation dictates," manager Bo Porter said. "If he's not able to get into the game today, he'll throw an extended bullpen and be ready to pitch Sunday.

Peacock won a spot in the rotation to begin last season but lasted only a month after going 1-3 with an 8.44 ERA. In his third stint with the club last year, he went 4-3 with a 3.67 ERA in his final 12 games, including nine starts.

"Peacock is the next in line, and I think he's going to pick up where he finished last year in September," general manager Jeff Luhnow said.

Grossman optioned after slow start to season

HOU@TEX: Grossman launches a three-run shot to right

HOUSTON -- Considering how well left fielder Robbie Grossman played in the second half of last season for the Astros, his struggles at the plate and in the field in the first two weeks of the regular season certainly came as a surprise to general manager Jeff Luhnow.

Grossman was optioned to Triple-A Oklahoma City following Tuesday's game after going 0-for-4 against the Royals. He was hitting .125 with six hits in 48 at-bats and has had a few misplays on defense. Grossman began the year as the starter in left field and hit second in the order.

"We saw what he's capable of doing at the end of last year," Luhnow said. "He's capable of being a very effective two-way player and we handed him that left-field everyday job and he played every game so far this year.

"He was given every opportunity. It was disappointing for him primarily, but also to us. I'm fully confident that Robbie will go to Triple-A and find his stroke and sort everything out and make it back here later this year and will help us win ballgames. I'm confident of that."

Grossman had two stints with the Astros last year in his Major League debut, hitting .322 with a .816 on-base percentage in his final 35 games after getting recalled in July. He hit just .198 in 28 games early in the year, but made some adjustments in the Minors that paid off. He'll have to do that again to get to back to Houston.

"I told Robbie, 'Look, we've been here before,'" manager Bo Porter said. "'You arrived last year and you struggled. You went back and you went down and took care of business and you came back and had a productive regular season.' I said, 'Treat this here the same. Go down there and take care of business and you'll be back here in short order.'"

His departure frees up some playing time for L.J. Hoes in left field, especially now that George Springer appears destined to start in right for the foreseeable future. Hoes played left field in the Minor Leagues with the Orioles and when he made his Major League debut.

"It's tough sitting over here feeling you can help the team and seeing those guy struggle," Hoes said of the Astros' early-season offensive woes. "You have to understand, it's what they want to do. It's a process and you have to have confidence in your teammates and their ability to snap out of it. Robbie's one of my good friends, and [Alex] Presley has been my teammate, and I've been talking to them to encourage them and telling them it's early in the season and to keep working and to break out of it."

Struggling Harrell designated for assignment

HOU@TOR: Harrell escapes inning unscathed

HOUSTON -- Lucas Harrell's time with the Astros appears to be over after the club designated him for assignment late Tuesday. He was told of the move after taking the loss against the Royals, falling to 0-3 with a 9.49 ERA in three starts this year.

Harrell's stock had fallen significantly since his breakout 2012 campaign in which he was 11-11 with a 3.76 ERA in 32 starts. He struggled to find consistency last year and wound up leading the American League in walks and losses while teetering between the rotation and bullpen.

"I think the reality is we gave him all of last year and three starts this year," general manager Jeff Luhnow said. "He deserved all that opportunity because he was good for us in 2012, but quite frankly we feel like at this point a change of scenery may be in the best interest of both Lucas and the Astros. Whether that means with another club or Triple-A, it's not something we feel like we can continue to do in Houston at this point."

Harrell told MLB.com it's probably time to move on, though the Astros could still wind up sending him to Triple-A.

"I was definitely surprised," he said. "I kind of saw it coming a little bit. Just in my starts, there was a ball off the wall here and plays here or there where it could have been a lot different. But they've got their plans and I understand that. I think it's best for both parties to kind of go their separate ways."

Manager Bo Porter said Harrell could benefit from going elsewhere.

"Sometimes situations run [their] course," he said. "Lucas Harrell is a Major League pitcher and we wish him nothing but the best. At the same time, sometimes a change of scenery is probably the best for a guy, and that's the point we got to as an organization."

Worth noting

• The Astros on Thursday are planning to call up left-handed pitcher Raul Valdes to replace lefty Kevin Chapman, who will be optioned to Triple-A Oklahoma City. Chapman appeared in six games and posted a 12.27 ERA in 3 2/3 innings, allowing five hits and five runs.