Starter Shelby Miller allowed only one hit and one walk in four scoreless innings while striking out three, retiring the last 11 batters he faced.
Joe Kelly wasn't nearly as successful. The Marlins tagged him for eight hits and scored their only runs of the game in a four-run seventh.
''I saw some positive things out of both of them,'' Matheny said. ''They just keep making this tougher and that's what they should be doing.''
Matheny also didn't give a time frame for making his decision.
For Miller, a former first-round draft choice, the outing was his best of the spring. He threw 31 of his 43 pitches for strikes and dispatched the Marlins with such efficiency that he needed to throw about 20 pitches more in the bullpen to complete his workload for the day after the Cardinals pulled him for Kelly.
Miller said part of the improvement came because he altered his mechanics to keep from tipping some pitches.
''I kept (my glove) more close to my body today and I think that helped me get more command of my pitches,'' Miller said. ''I felt I threw the ball down today and I felt good.''
While the statistics weren't pretty for Kelly, he wasn't too disappointed with the results. All six hits he surrendered in the seventh were singles, some on broken bats.
The Marlins had two hits to begin the inning, but Kelly almost escaped the inning unharmed after getting Justin Ruggiano to ground into a double play. Kelly had two strikes on Adeiny Hechavarria before allowing four consecutive singles.
''It takes a lot of singles to score and obviously that's what they did,'' said Kelly, who has been working on generating more ground balls. ''That kind of style of pitching you are going to either go deep into the ballgame quick, with guys hitting grounders or sometimes they are going to get through because you are pitching to contact.
Marlins' starter Jacob Turner had battled control issues all spring until Tuesday. In his longest Grapefruit League outing Turner went five innings, allowing seven hits and four earned runs.
Turner only walked one batter, though he did uncork a wild pitch.
''My mechanics felt good and I was able to throw the ball where I wanted to,'' Turner said. ''I think there were some pitches in the middle innings that I left a little farther up than I would have liked, but other than that I felt pretty good.''
The Cardinals added two more runs in the fifth courtesy of a bunt single by Kelly and a couple more hard-hit balls. Matt Carpenter's double to right field brought in both runs. Kelly singled in another run in the sixth.
NOTES: Cardinals - St. Louis released utility infielder Ronny Cedeno after Tuesday's game.. ...Third baseman David Freese once again missed the game with back soreness. Cardinals' manager Mike Matheny expects Freese to be back in the line-up on Thursday. ... Marlins - Greg Dobbs played the field for the first time this spring. He suffered a calf injury during the first couple games while he was serving as a designated hitter. Dobbs went 0-for-3 on Tuesday. ... Kevin Slowey will be one starting pitcher in an intra-squad game on Thursday's scheduled off-day for the Marlins.
Link to box score: St. Louis 5, Miami 4