With the Cardinals closer questions apparently being answered by Edward Mujica, who has converted all 3 of his save chances, the Cards are still looking to shore up the rest of their bullpen.
As of today, the Cardinals have a bullpen ERA of 5.37, last in the majors. Most of this can be attributed to Mitchell Boggs, who struggled when placed in the closer role and has yet to sort things out. If you remove his 12.46 ERA in 8.2 IP, the Cardinals bullpen ERA is a more respectable 3.74.
However, the problem is that Boggs was expected to play a major role in the Cards pen. He was the main setup guy last season and, before Jason Motte's injury, he was expected to fill that role again this year.
With his struggles, Trevor Rosenthal has had to step up into the setup role, a role that he finally appears to be getting use to. After, blowing two saves a little over a week into the season, Rosenthal has managed 5 holds since then. He is still trying to find himself, as he hasn't been as dominant as he was last year (15 hits allowed in 11.2 IP), but he seems to be acclimating to his new role.
But, with Mujica moved to the closer role and Rosenthal handling the 8th innning duties, the Cards are still looking for a 7th inning guy. Boggs could be that guy, if he gets things sorted out, but if he doesn't, the Cardinals will look at other options.
One such option, as Bernie Miklasz points out, it to swap the roles of Joe Kelly and Boggs, make Kelly the 7th inning setup guy and move Boggs to the long-relief role. This would give Boggs time to sort out his issues while giving Kelly a chance to show what he can do in a more prominent role.
Another option that Miklasz discussed would be to move Lance Lynn to the pen and call up a starter from AAA (Michael Wacha or John Gast). However, the Cardinals starters have been just fine, posting a 2.38 ERA (tops in the majors), so its unlikely that Mike Matheny would want to mess with his rotation. If it ain't broke....
Finally, the Cardinals could call up Carlos Martinez to fill a setup role. Joe Strauss speculated that this could happen in a recent interview. Martinez has closer type stuff, but just recently made his first start of the minor league season after arriving in the U.S. late due to visa issues.
My feeling is that the Cardinals should try Kelly in a setup role and move Boggs to long relief. I think this is the best option at the current time, as it gives Boggs time to sort out his issues while giving Kelly a bigger role. If things don't work out, then Plan B should be to give Carlos Martinez a try.
In any case, there is still work to be done to sort this all out. The Cardinals could always shock the world and pull off a trade. Strauss tweeted recently of a possible April/May trade (please, anyone but Carlos Marmol!!!). But, until that happens, the Cards will have to make due with what they've got.