KISSIMMEE, Fla. -- The decision to release left-hander Zach Duke and outfielder Jack Cust, which certainly came as no surprise considering how much the pair struggled this spring, has brought a tad more clarity to the roster composition.

General manager Jeff Luhnow said that Tuesday's decision to cut ties with the veterans had more to do with the emergence of some younger players who are battling for the same spots on the roster that Duke and Cust were trying to win.

"What we're talking about here is [that] the young players have stepped up and demonstrated they're ready to be on this ballclub," Luhnow said. "We had to have a contingency plan, which is to have some veterans around in case the young players showed they weren't ready. It didn't happen."

With Duke out of the picture, the race for the final two spots in the rotation is down to three right-handers: Livan Hernandez, Jordan Lyles and Kyle Weiland. Hernandez, a non-roster player, appears to be a favorite for a spot, which would leave youngsters Lyles and Weiland battling for the final slot.

Lyles, 21, is 0-2 with a 6.32 ERA in four games (three starts this spring), and Weiland is 2-0 with a 2.25 ERA in five games (three starts). Weiland was acquired during the winter from the Red Sox, along with shortstop Jed Lowrie, in exchange for closer Mark Melancon.

Duke's departure also leaves Wesley Wright and Fernando Abad as the only two left-handed bullpen candidates. Manager Brad Mills would like to carry two lefties in the bullpen if possible, so either Wright or Abad -- or both -- will make the club on Opening Day.

The Astros have 39 players left in camp.

"We're still sorting through it, and we haven't made any final decisions yet," Luhnow said. "Lyles was good [on Monday], and I got a chance to watch Weiland a bunch of times. We're happy with the mixing and matching with some of the youngsters in the rotation, and we're going to feel pretty good about it."

Cust was signed to provide the Astros with a left-handed option in the outfield and was competing with fellow left-handed hitters Travis Buck and J.B. Shuck and right-handed bat Justin Ruggiano. All three outplayed Cust this spring.

"In the outfield we're pretty heavily left-handed after we moved [Jason] Bourgeois, but Buck and Shuck have both played well, and Ruggiano has played well," Luhnow said. "These guys all have a chance to add value to our team this year."

Duke, who was on a Minor League contract, went to camp being viewed as both a reliever and a starter, but he struggled. He appeared in four Grapefruit League games, making three starts, and went 0-3 with an 11.30 ERA.

Mills said that Duke's location wasn't where the club needed it be for him to start, and his breaking ball wasn't good enough for him to pitch out of the bullpen.

"Any time you have to give a release to somebody, that's a difficult thing to do," Mills said.

Cust, who signed a one-year, non-guaranteed, $600,000 contract with an option for 2013, had only one hit in 25 spring at-bats and never appeared in the outfield because of discomfort in his left elbow.

"The thing with Cust ... We weren't able to use him in the outfield all of spring, really," Mills said. "He got to play a couple of innings of a 'B' game out there, and the bat wasn't there and [it wasn't] what we had hoped. We know he's got the ability to do that, but it wasn't able to show in the 30-something plate appearances we gave him."