A stress reaction is much like a stress fracture, but at an earlier stage. According to Orthopod, both issues occur “in response to repetitive overloading of a bone,” in this case, Wacha’s shoulder. The difference is the fracture will show up in imaging, like X-rays or MRIs; the reaction only shows up as increased activity, as the injury hasn’t gotten to the point where a fracture can be seen.
The problem is many times a stress reaction shows up in feet. The site also notes that the injury will require 6 weeks of rest before it gets better (he also notes that a cast is typical, but that is for the foot and not the shoulder).
That is in line with McCarthy’s history with the injury.
McCarthy has been on the DL 3 times in his career with stress reactions, 2 of them in May (2011, 2012). He explained that his shoulder felt “soreness and fatigue” and it normally happened at the same point in the season. X-rays have always returned negative for a fracture. Each time he’s gone on the DL, he required at least 6 weeks to recover.
With little known in baseball about stress reactions, the Cardinals should figure that they will be without Wacha at least 6 weeks; a more cautious approach would be to keep him out closer to 8 since they have such high expectations for the pitcher. He’s a key player for the future of the franchise and a slow recovery is the best for him and the organization.
Figuring 2 months without Wacha would definitely impact the team this season. The team is sitting back 6.5 games in the standings and in a tie for second place in the Central; the team is also on the outside of the Wild Card by 2.5 games. If the team is willing to make a move, they will need to do it soon. With Wacha, Joe Kelly and Jaime Garcia out and question marks around the health of Lance Lynn (blisters), Shelby Miller (back), and Adam Wainwright (elbow), the team will need to find a proven starter to go along with a boost to the offense.
With Oscar Taveras being promoted earlier today, speculation of a big trade will start up again; rumors of a “Taveras Showcase” will start up. David Price and Jeff Samardzija will probably both be tied to the Cardinals since they are the big names on the market. The Cardinals will probably look to a smaller deal for someone like Jake Peavy, Jason Hammel, or Jorge de la Rosa; someone that won’t require Taveras.
Another option would be to acquire a closer so Trevor Rosenthal could move to the rotation; Rosenthal is vocal about his desire to start, and moving him now could start the transition without a large increase in innings.
Whatever the Cardinals do, they need to take it slow with Wacha; he's too important and his injury is too rare to try to push him back too quickly.