Should we be concerned about Mike Matheny's overuse of Trevor Rosenthal?

After a bumpy start to the season, Trevor Rosenthal has settled into his setup role nicely.  Since April 24th, he has allowed only 1 unearned run in 9.2 IP.  He has seen his ERA drop from a high of 5.00 on April 17th to the current low of 2.42.

However, despite his run of success, there is one thing that concerns me, Mike Matheny has been using Rosenthal A LOT.

Rosenthal has appeared in 23 of the Cardinals first 44 games, which puts him on pace for about 85 appearances for the year.  While it is not unheard of for a team to use a pitcher that much, its rare for a team to do that to someone under the age of 25.  Since 2000, only 4 pitchers under the age of 25 have appeared in more than 81 games in a season:  Oscar Villareal, Matt Capps, Jonathan Broxton and Joe Smith.

Oscar Villareal appeared in 86 games in his rookie season at the age of 21.  He reported a sore elbow the next spring and later developed rotator cuff issues and has not been the same pitcher since.

Matt Capps appeared in 85 games as a 22 year old rookie in 2006.  He then appeared in 76 games in 2007.  In 2008, he experienced shoulder bursitis and missed nearly two months.

Jonathan Broxton appeared in 83 games in 2007 at the age of 23.  He pitched in 70 and 73 games the next two seasons before finally showing signs of wear and tear in the 2010 season, when he posted a 4.04 ERA in 64 games.  Then, in 2011, he experienced elbow issues, which knocked him out for 5 months of the year.

Joe Smith was 24 when he pitched in 82 games in 2008.  He then missed a little more than a month in 2009 with a strained rotator cuff.

So, of the 4 pitchers I found under the age of 25 who pitched in more than 81 games, all 4 of them experienced arm injuries following that season.  Two of them experienced those injuries the next year and the other 2 experienced their injuries after continued overuse.

I admit this is a small sample size, but could this be the relief pitcher equivalent of the "Verducci Effect"?

For his part, Matheny is not concerned, saying "He’s one of the physically strongest guys we have. If we find it’s affecting him, we change course. He prepared to be a starter this year. He prepared for 200 innings. We’re not going to give him that.  But we’re going to give him an opportunity to help us win. And that’s what he’s been helping us do.”

Sure, Rosenthal prepared to be a starter this year, but there is a big difference between throwing 200 innings as a starter--when you are on a set routine, pitching every 5th day--and throwing 85+ innings, with no routine and sporadic rest. Rosenthal recently pitched in 3 straight games and 4 out of 5 days before getting yesterday off.

I can understand why Matheny is using Rosenthal so much.  The Cardinals bullpen has been a mess and Rosenthal has been one of the few bright spots in the pen. Matheny wants to win and Rosenthal gives him the best chance of doing so.

However, Matheny needs someone else to step up and lessen the burden on Rosenthal before its too late.