LAKELAND, Fla. -- Brett Wallace, who began last season as the Astros' Opening Day first baseman and was competing for a backup infield spot this spring, was optioned to Triple-A Oklahoma City in a series of roster moves announced by the club Sunday.

The Astros also announced Rule 5 Draft pick and shortstop Marwin Gonzalez had made the Opening Day roster, along with outfielders J.D. Martinez and Brian Bogusevic, second baseman Jose Altuve and catcher Jason Castro.

Astros general manager Jeff Luhnow said the five players -- all of whom are on an active Opening Day roster for the first time -- were notified prior to the Grapefruit League finale against the Tigers by manager Brad Mills.

"It's one of the fun things we get to do," Luhnow said. "We bought them into Brad's office and let them know."

Wallace is hitting .260 with one homer and seven RBIs in 50 spring at-bats. He played both first and third base this spring in an effort to increase his versatility, but he will start the year in the Minor Leagues and play third base.

"I'm obviously disappointed," Wallace said. "Everybody wants to be on the team, but I'm going to go down there and take it, and work hard and try to get back here as soon as I can."

Gonzalez is hitting .182 this spring, but he's shown the ability to play terrific defense, including starting a nice 6-4-3 double play on Sunday. As a Rule 5 Draft pick, he'll have to stay on the 25-man roster all season or be offered back to the Cubs.

Luhnow said the decision to keep Gonzalez on the Opening Day roster has nothing to do with the health problems of starting shortstop Jed Lowrie, who's nursing a sprained thumb.

"It's really independent," he said. "Marwin has done a terrific job defensively and shown he's capable of taking Major League at-bats and had a couple of good plays in this game today. He's ready, and he's a valuable asset for us, and we'd like to have a guy like that in our organization. To do that, we have to keep him on the [25-man roster] and we're going to start the year with him on the [25-man roster], and hopefully it will work out."

The moves leave the Astros with 31 players in camp ahead of Wednesday's deadline to set the 25-man roster. Considering left-hander Sergio Escalona is still on the roster and is facing Tommy John surgery on Wednesday, the Astros have five remaining cuts to make.

Luhnow said more roster moves would come on Monday.

Wallace, acquired by the Astros at the Trade Deadline in 2010, got off to a terrific start at the plate last April, but slowly watched his average decline. He was sent to the Minor Leagues at the end of July and returned to the big league club when rosters expanded in September, but Carlos Lee was entrenched as the starter at first and rookie Jimmy Paredes had taken over at third.

Wallace, who played third base in college, will remain with the team through this week's exhibition games before reporting to Oklahoma City.

"We thought he adapted to third base very well this spring after not playing there for a few years and did a real good job there," Mills said. "Offensively, I thought we saw some progression this spring. He worked in the offseason in the weight room and on the field as far as swinging the bat and defensively, and we were able to see that this spring."