The roster, which must be set by Wednesday, could also be impacted by the health of center fielder Jordan Schafer (sprained left hand) and shortstop Jed Lowrie (sprained right thumb). They both could start the season on the disabled list.
The Astros are also going to open the season with both of their Rule 5 picks on the roster in Cruz and infielder Marwin Gonzalez, who was informed on Sunday he had made the Opening Day roster. They must remain on the 25-man roster all season or be offered back to their former teams.
Lyles came into camp as a favorite to win a rotation spot, but he struggled this spring. After making his Major League debut last year at age 20 and appearing in 20 games, including 15 starts, he scuffled to an 0-3 record with an 8.41 ERA in five games (four starts) this spring.
With Lyles beginning the year in the Minors, Weiland and Harrell will occupy the final two spots in the pitching rotation, joining Wandy Rodriguez, Bud Norris and J.A. Happ. Harrell was originally ruled out of the running for the rotation two weeks ago, but he pitched well his last few outings.
"You can look at the move we made today, and that answers that piece," Luhnow said. "There's still one or two more pieces that we're still debating. As far as Harrell being built up as a starter, he's on the club and you can assume [he's in the rotation]."
Harrell is lined up to start the second game of the season, on Saturday against the Rockies.
"It would be logical to assume that, but I don't want to make any announcements where people are going to pitch," Luhnow said.
Astros manager Brad Mills said after the game that the rotation will be Rodriguez, Harrell, Norris, Happ and Weiland.
Carpenter, 26, made his Major League debut with the Astros last season and made 34 appearances for the big league club, posting a 1-3 record with one save and a 2.93 ERA. He pitched at both Double-A Corpus Christi and Triple-A Oklahoma City last season, combining to compile a 1.91 ERA in 33 Minor League appearances.
Cruz, 25, was the first player selected in the 2011 Rule 5 Draft after spending last season in the Mets' organization. At the Class A Advanced and Double-A levels, Cruz combined for a 5-3 record with seven saves and a 3.89 ERA in 44 appearances.
Harrell, 26, went 1-0 with a 2.16 ERA in seven spring appearances. He made six appearances for Houston as a September callup last season, including two starts, and went 0-2 with a 3.46 ERA.
Rodriguez, 27, saw his first extended Major League action in 2011 with Houston, appearing in 47 games and finishing with a 2-3 record, 3.96 ERA and 57 strikeouts in 52 1/3 innings.
Weiland, 25, made his Major League debut in 2011 with Boston, appearing in seven games (five starts) and posting an 0-3 record and a 7.66 ERA. Acquired by the Astros in a trade last December, he went 2-1 with a 2.86 ERA in six games (four starts) this spring.