KISSIMMEE, Fla. -- Brad Peacock's father, Jerry, drove his son's pickup truck back to Houston recently in anticipation of watching him play at Minute Maid Park next week. Even better, he's going to get to see his son make only his third Major League start.

The Astros tabbed Peacock for the fourth spot in their pitching rotation Thursday morning, setting the 25-man roster in the process by informing right-hander Alex White he would work in long relief. Peacock won the spot by allowing just two earned runs in his final 16 innings in Grapefruit League play.

Houston manager Bo Porter told Rule 5 Draft pick Josh Fields he had made the club, along with fellow relievers Hector Ambriz and Rhiner Cruz. That means veteran Edgar Gonzalez is headed for Triple-A Oklahoma City.

Peacock, acquired along with starting left fielder Chris Carter and catcher Max Stassi by the Astros in last month's trade that shipped Jed Lowrie to Oakland, will start Friday's game at Minute Maid Park against the A's -- his former team.

"It was unbelievable," said Peacock, who pitched all 2012 in Triple-A after appearing in three games (two starts) for the Nationals in '11. "I never thought this was going to happen coming in here this year, but I worked hard this offseason, and it paid off."

The Astros remain high on White, and he'll wind up getting a chance to get back into the rotation the first time a slot becomes available.

"Peacock's pitched so well, and he deserves that spot," White said. "I have no problem going to the 'pen, because he earned it and I'm excited for him and myself."

Only seven players on the Astros' Opening Day roster -- pitchers Bud Norris, Lucas Harrell, Wesley Wright and Rhiner Cruz, infielders Jose Altuve and Marwin Gonzalez and catcher Jason Castro -- were on the team's 25-man roster to start last year. The average age of the roster is 27.6 years.

Porter was pleased to have to decide between two good options to put into the rotation, which is a sign the Astros' improved starting pitching depth.

"[Peacock] did a tremendous job," Porter said. "I talked to him and White and told them I thought we deserved to have six starters make the rotation, and I'm a big believer in carrying a long guy for the flexibility of saving your bullpen. I told White, 'You're going to get your innings.' It's a competition that's not just going to stop at the end of Spring Training. It will continue through the course of the season."

Porter likes the makeup of the bullpen, too. Jose Veras will be the closer and the eighth-inning role will initially be determined by matchups, though the skipper eventually wants someone to fill the role full-time. In addition to White, Porter said Ambriz and Cruz can pitch multiple innings from the right side and Xavier Cedeno from the left side. Cedeno and Wesley Wright are the two lefties.

"I love the fact we have two lefties down there who are capable of getting out some of the better left-handed hitters in our league," Porter said. "We also have a long guy, we have a good mix of guys who can pitch in the back end. I'm pretty confident in our bullpen."

With Norris (Sunday), Harrell (Tuesday) and Philip Humber (Wednesday) pitching against the Rangers in the season-opening series, Peacock will start Friday against his former team, the A's. Erik Bedard is scheduled to pitch Saturday against the A's, with Norris throwing the homestand finale on Sunday.

"It's great," Peacock said. "I'm really excited to face them. Some of my buddies are on that team, and I'm just ready to go."

Peacock was selected by Washington as the No. 41 overall pick in 2006 and was shipped to Oakland five years later in the Gio Gonzalez trade.

Peacock was 12-9 with a 6.01 ERA in 28 games (25 starts) at Triple-A last year following a terrific 2011 season. In that campaign, he made his Major League debut with the Nationals and was 2-0 with a 0.75 ERA in three games (two starts).

"For the whole staff, he just looked like he got stronger each and every time he got the ball," Porter said. "The outings got strong, the stuff got more crisp and the breaking ball really, really starting to get late depth. It was good to see. I have history with him and I saw him really, really good in 2010, and like I told him, 'He looks like the guy I saw in 2010.'"

Fields, 27, had never pitched above Double-A before appearing in 10 games last year at Triple-A Pawtucket in the Red Sox organization. The Astros took him with the first overall pick of the Rule 5 Draft and will plug him into the bullpen.

"I've had a few of those meetings before [with the manager], and none have gone that way," Fields said. "I feel blessed and thankful the Lord gave me this opportunity, and I'm pretty excited to be a part of this team."