First off, who gave you the diagnosis? It obviously wasn't an orthopedic, so that diagnosis is weak at best. General practice docs will generally err on the side of caution and overestimate recovery times.
See the ortho and you'll get a much more straight forward, realistic answer. Is it really RC inflammation/tendinitis? Or is it something more severe like a Bankart or labral tear? Or something even more minor?
Generally speaking, a steroid/cortisone injection does not require a month of rest. With my patients, we generally give them 2-3 days off before resuming normal activity. Physical therapy would also be a huge help, as I'm sure Smashing Machine would agree.
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