Back injuries are diagnosed using clinical exam. They are most often detected when athletes complain of soreness, pain, and tightness in the back. Typical strains include mild pain and almost constant discomfort. Tests such as X-rays may be used to distinguish muscular strains from bone problems, and an MRI or CAT scan may confirm the existence and severity of the injury.
Treatment for the injury is simple. As a strain is a mild muscular injury, it is treated with rest, ice, and anti-inflammatory drugs. Most importantly, the athlete must try to limit all use and movement of the back, because it is far more difficult to immobilize than an extremity. Sometimes athletes try to play through back pain; in almost all cases, the pain does not go away and eventually worsens.
The recovery period may vary from 1-2 days to 2-3 weeks. In McGwire's case, the injury occurred very early in the season, and he chose to be very cautious and rest his back for a longer period of time. Even though he is taking this time off, it is likely that he will require more days off during the season than he has in the past; he may require a few days off at some point just to rest his back. Like most back injuries, this is a nagging problem, and as the season winds down, he may play through some minor pain.
McGwire will be affected in two ways. First, due to soreness and the need for rest, he will probably miss more games than he has over his last two seasons. Second, he may lose a tiny amount of strength and mobility in his back that would slightly lessen his power. Due to the nature of strained backs, there is a good possibility that he will reinjure his back if he does not correct the motion that caused the injury. Because that motion involved swinging a bat, even if he rests his back all of next off-season, there is still a chance he will suffer from soreness when he returns to play.