Background <p>The impact of tear etiology (traumatic vs. degenerative) compared to tissue quality on outcomes after arthroscopic rotator cuff repair (ARCR) remains debated. This study aimed to evaluate the influence of preoperative fatty infiltration (FI) and injury mechanism on postoperative functional recovery.</p> Methods <p>We retrospectively reviewed 386 patients undergoing primary ARCR for isolated full-thickness supraspinatus tears. Patients were stratified by etiology into traumatic (<i>n</i> = 132) and degenerative (<i>n</i> = 254) groups. FI was graded using the Goutallier classification. Functional outcomes were assessed using Visual Analog Scale (VAS), American Shoulder and Elbow Surgeons (ASES) score, and University of California, Los Angeles (UCLA) score. Multivariate regression analysis was performed to identify independent predictors of poor functional recovery.</p> Results <p>Advanced FI was significantly more prevalent in the degenerative group (<i>p</i> = 0.013). Regardless of tear etiology, patients with advanced FI exhibited significantly inferior postoperative functional scores (VAS, ASES, and UCLA; all <i>p</i> &lt; 0.05) and higher rates of joint stiffness compared to those with early FI. Multivariate analysis identified advanced FI (OR = 3.32; <i>p</i> &lt; 0.001) as a significant independent risk factor for poor recovery, whereas injury etiology did not independently predict outcomes.</p> Conclusions <p>Preoperative fatty infiltration is a more critical determinant of functional recovery than injury etiology. The perceived clinical superiority of traumatic tears likely reflects the better baseline muscle quality in this patient population. Consequently, surgeons should prioritize the comprehensive assessment of individualized muscle quality when establishing surgical expectations and counseling patients.</p> Level of evidence <p>Level III, Retrospective cohort study.</p>