Although rotator cuff injures are often associated with a limited range of motion and muscle weakness, being able to conduct pain-free and efficient performances of the activities as part of daily living seems to be more important for patients. The aim of this study was to investigate the correlation between two questionnaires—the disease-specific, subjective questionnaire termed the Western Ontario Rotator Cuff Index (WORC), and the shoulder-specific, subjective-objective questionnaire Constant–Murley score (CMS), with the objective assessment of external rotator muscle strength, and the subjective assessment of pain according to the visual analog scale (VAS) in patients after arthroscopic rotator cuff repair. The study was carried out among 47 patients twice—6 and 12 months after surgery, respectively. All patients completed the WORC, the CMS, and the VAS. Isokinetic evaluation of the external rotators was performed using the Biodex 4 ProSystem. The correlations of all assessed muscle strength parameters with both the CMS and the WORC were found to be statistically significant, being mostly average during the 1st examination and mostly strong during the 2nd examination. There was a significant improvement in all assessed tools as a result of the undertaken rehabilitation. There were weak correlations present between changes in the WORC and changes in the external rotator muscle strength, with correlations between WORC-Sport and EXT900-AVERAGE-POWER and PEAK-TORQUE also being found statistically significant. Correlations of changes in the CMS scale with changes in the external rotator muscle strength were weak and statistically insignificant. It seems that the WORC questionnaire can be recommended more for the population after rotator cuff repair, which allows for a reliable assessment of patients’ ability to function and its changes in various areas of life, and at the same time does not require a direct assessment by a clinician or researcher.