Strains of the oblique muscles and their tendons occur when the muscles are most at risk in sports when the athlete is involved in movements that extend and twist the trunk (butt) of the body. This extension and rotation is required to create force in the pitching motion, and as the athlete completes the motion by following through, the abdominal muscles flex the trunk and the internal and external muscles on the opposite side counter the trunk rotation.
As athletes have become bigger and more powerful and this movement has become more dynamic, the incidence of injury to the oblique muscles has increased. As large of frame as Masterson has, it is no surprise that this type of injury has occurred.
Rest is going to be the major injury management for Masterson. Rest from activity will allow the injured muscle to heal. With this particular muscle/tendon, healing is a slow process and subject to setbacks because movements of the trunk during activities of daily living can irritate this muscle. If Masterson reaches too far for one of his wife's cookies, he could miss a few more starts. But once Masterson has no pain for several days, then gentle, progressive stretching exercise can begin.
Since muscles heal by the formation of scar tissue, which is only 70% as strong as the tissue it replaces, it is important that Masterson is not rushed back, which could lead to a re- occurrence of the injury. With less than a month left in the season, a re-occurrence of the injury would not be a major set-back due to the fact that Masterson can will have the entire offseason to heal.