Jason Motte's injury this spring left the Cardinals scrambling to find a replacement at closer. Edward Mujica has filled in admirably so far, but he is set to become a free agent after this season and, with Motte likely starting next year on the DL, the Cards will likely face the same closer issues next year.
The question then is who will become the Cardinals next great closer? The guy that Mike Matheny can hand the ball to in the 9th inning for the next several years and not have to worry (or at least not too much).
Based on the evidence so far, it seems likely that either Trevor Rosenthal or Carlos Martinez will take over the closer role, either by the end of this year or via a spring training battle next year.
Rosenthal would seem to be the logical choice to take over as closer. Despite spending most of his minor league career as a starter, the Cardinals used him as a reliever down the stretch last season. In addition, after stating that he would compete for the 5th starter role in the spring, the Cards barely gave him a chance before moving him back to the pen.
Thus, it appears that the Cardinals have already made up their minds that Rosenthal is best used as a reliever. His high 90's fastball, hard curveball and solid change give him three weapons to use late in games and, after struggling early in the season, he appears to have hit his stride.
As for Carlos Martinez, he too spent all of his time in the minors as a starter. However, when the Cards needed to shore up their bullpen recently, they bypassed AAA options such as Victor Marte and Eric Fornataro and chose Martinez from AA. After missing the start of the season due to visa issues, Martinez had only pitched 11.2 IP prior to his callup. If the Cardinals were content with developing him as a starter, they would have left him at AA.
What's even more telling for both Martinez and Rosenthal is that the Cards are going to need another starter next year. Jake Westbrook is set to become a free agent after this year and its unlikely that the team will offer him an extension.
However, with Michael Wacha pitching well at AAA and the emergence of John Gast as a legitimate top 10 prospect for the Cards, the team must feel confident that one of those two can take over for Westbrook next year, giving them the luxury of developing Rosenthal and Martinez as relievers.
Normally I do not like when a team takes a pitcher who could develop into a frontline starter and convert him into a reliever, but with the Cardinals pitching depth, the team appears to be trying to find spots for all their young guns.