Often the range of movement in a shoulder with a small to medium size full thickness supraspinatus tear is preserved. Complete lack of external rotation may be a sign of adhesive capsulitis.
A painful arc (pain in the antero-superior aspect of the shoulder, in 30-90 degrees of abduction) may be present. Rotator cuff strength should also be assessed. Supraspinatus abducts the shoulder at 30 degrees, subscapularis is an internal rotator of the shoulder and infraspinatus is an external rotator.
Any surgical or invasive procedure carries risks. Before proceeding, you should seek a second opinion from an appropriately qualified health practitioner.