Is a rotator cuff injury a serious injury?

Yes: At your age, a rotator cuff tear would be a very serious injury. It is extremely rare for a 31 year old to have a rotator cuff tear. A strain of the rotator cuff, however, usually resolve on their own without too much worry or too much long term difficulties. If may be time to see your friendly neighborhood orthopedic surgeon!

It depend: What kind injury, if minor inflammations it is not, if you have a tear in it is need surgery.

Yes: It can significantly effective quality of life. Sometimes people have tears and have no symptoms or deficits. For most people it is quite painful and annoying.

Yes.: If it's torn, it cannot heal itself, and may require surgical attention.