Now, Carpenter has proven he is good enough to stay.
"I want him to see that it's a compliment that he's done a very nice job making that transition," Matheny said. "The rest of the baseball world, I think, has been overlooking what this kid has been able to do offensively and defensively and all the way around. He's done a terrific job. He's been a difference-maker for us."
Told of Matheny's high praise, Carpenter responded: "That's quite a compliment. I'm very happy the progress that I've made over there. Obviously, I'm proud of the way I've been offensively. But defensively, for me, that was my main goal. If I went over there, I wanted to be somebody they could count on. I didn't just want to be a guy who would be there for seven innings and then [it be], 'Get him out of there because he's a mess.' I've been real happy with the way it's gone, and hopefully I can keep that going."
The numbers -- on both the offensive and defensive ends -- provide some substance to Matheny's bold claim. Entering Wednesday, no second baseman in baseball had a higher WAR (wins above replacement player) than Carpenter's 3.4 mark, as calculated by fangraphs.com.
On the offensive end, Carpenter ranks second among National League second basemen in batting average (.327), and first in on-base percentage (.411) and on-base plus slugging (.889). No leadoff hitter has a higher on-base percentage than Carpenter does from that spot.
His ultimate zone rating -- a defensive statistic that compares a player to an average defender at that position -- is 3.3, the third-highest in the NL.
"He's made some tough plays look very easy," Matheny said. "I give [third-base and infield coach] Jose Oquendo a lot of credit for the in-game instruction, constantly point him in the right direction or giving him a reminder of something that might be going on, whether it's a bunt defense or a cut-off situation.
"That's the one variable that we knew we had, was the work [ethic]. He's not afraid to work. He'll outwork anybody. You take talent and you mix that together, you're set up to give yourself a good chance. But he's done even more than we thought."
Recognition outside of the Cardinals' clubhouse and fan base has been slow to develop for Carpenter. When Major League Baseball released its first All-Star voting tabulations, Carpenter was ranked fourth among NL second basemen. He moved up to third in the voting, according to the most recent release of numbers.