Termed ''Plan B'' earlier in the week by Matheny, Lynn retired the final eight batters he faced in his first spring start, which the Cardinals lost 4-3 to Houston.
''I wanted to go back out, but I already had 40 pitches,'' said Lynn, whose fastball hit 96 mph on the stadium radar gun.
Lynn was destined for the bullpen at the start of the spring, but the Cardinals announced earlier in the week that he would be treated like a starter until Carpenter shows he can return to the rotation.
The only place Lynn really struggled Wednesday was in his warmup tosses, which he finished too early.
''I have to find the routine I'm going to use,'' Lynn said. ''In Triple-A, I usually started earlier. I don't know whether I just got loose faster or what it was. Next time I will go a little later so I won't have to wait around.''
Lynn made his major league debut in 2011, going 1-1 in 18 appearances (two starts) with a 3.12 ERA. He appeared in 10 postseason games and went 2-0. His only spring appearances before Wednesday came in in relief.
Against Houston, Lynn allowed a pair of one-out singles in the first inning before striking out Jack Cust and inducing a lazy fly ball from Jimmy Paredes. He cruised through the final two innings without allowing a baserunner.
''He looked great today,'' Matheny said. ''He had great velocity, good movement on the ball, and you could just tell (Houston) had awkward approaches on their swings. Those are all positives.''
A couple hours before Lynn's start, Carpenter threw for the first time in nearly a week, playing catch in the outfield with Jake Westbrook. Carpenter has been experiencing neck pain caused by a bulging disc.
The former Cy Young winner had to be scratched from his March 12 start and has not appeared in a game this spring.
''I'm going to continue to go in the right direction,'' Carpenter said. ''We'll move forward and see what happens.''
Matheny said he's yet to be given a timetable for Carpenter. Lynn is scheduled to start again Monday against Atlanta.
Matt Holliday, David Freese and Yadier Molina strung together consecutive RBI singles in the third. The trio managed consecutive singles again in the fifth. Holliday finished 3 for 4, raising his spring average to .462.
The Astros scored their four runs in the sixth, with all of them charged to Adam Ottavino.
NOTES: Alex Cora saw his first action of the spring at first base, going 1 for 3 in place of Lance Berkman, who had what Matheny termed an ''achy knee.'' Berkman will also receive a previously scheduled day off when the Cardinals travel across the state to face Boston on Thursday. Matheny expects Berkman to return to the lineup for Friday's home game against Miami. ''Lance really wanted to play,'' Matheny said. ''To give him another day isn't going to hurt him.'' ... Matheny also chose to give Carlos Beltran a routine afternoon off. The Cardinals' right fielder played in consecutive games Monday and Tuesday for the first time after battling an illness for much of the spring.
Houston 4 at St. Louis 3: link to boxscore