Muscle edema is a known marker for acute muscle trauma. Its early appearance in disease progression has been previously documented in animal models. On the other hand, muscle atrophy and fatty infiltration generally develop in sub-acute and chronic phases of rotator cuff injury. There is currently a lack of extensive description of muscle edema, particularly following tendon retraction, in human cases. This study provides new insights by describing the characteristics of rotator cuff muscle edema in humans, emphasizing the difference between purely acute lesions and acute-on-chronic ones. It demonstrates that muscle edema appears earlier and more prominently in younger patients with acute injuries, while the presence of fatty infiltration indicates an extension of pre-existing, chronic lesions. It also suggests that early MRI imaging is critical in differentiating these conditions, which can guide surgical planning and treatment.