The St. Louis Cardinals had already decided to put Trevor Rosenthal into the bullpen before Saturday's announcement that Jason Motte was injured and would likely start the season on the disabled list, but now that their closer is dealing with an elbow strain it looks like that probably would have happened no matter what.
Which is not to say that he'll close. According to Jenifer Langosch's report (linked above), Mitchell Boggs, briefly the closer in 2011, will return to the job to start the season. That leaves Edward Mujica and Rosenthal in the eighth inning job, strengthens Fernando Salas's position in the bullpen, and means that Joe Kelly and Shelby Miller could very well both start the season on the Cardinals' active roster.
The Cardinals have a lot of pitching prospects, and they'll be okay without Motte for a while. But I'd hate to understate just how good he was in 2012—he struck out nearly 11 batters per nine innings, carried a K:BB ratio over five, and (if you like that sort of thing) led the National League in saves. If we're very lucky, Trevor Rosenthal will pitch about that well in the bullpen this season.