Rotator cuff

First of all, what is a rotator cuff?
Rotator cuff muscles are 4 small muscles that help to hold the arm in the in the shoulder socket while bigger muscles perform movement. The shoulder is a ball in socket joint which means it is highly mobile but somewhat instable. That is why these muscles are required to add additional stability.
How do they tear?
Rotator cuff tears usually come about when the shoulder sustains a sudden stretch or is over exerted such as a heavy lift or throwing manoeuvre.
 
Does a tear repair?
Usually not, that’s why sometimes tears in the rotator cuff can appear on ultrasound or MRI imaging even years after they were sustained. That’s ok though, usually the tear only makes up part of the muscle, so the rest of the muscle and the other surrounding muscles can compensate.
Most of the time people with a rotator cuff tear wont even know its there.
In one study of baseballers, 83% of players with a tear in one or more rotator cuff muscles on ultrasound did not have any symptoms or pain.
 
What do I do if I have a sore shoulder?
As mentioned above, a rotator cuff tear may be present for years without much issue, the problem will come when you decide to try something strenuous with the shoulder that it isn’t used to. This usually comes at the start of a sports season, renovating a house or some other infrequent or once off bout of exercise. 
Given that rotator cuff tears are so common, even in people without pain. It won’t be all that helpful to get an image of the sore shoulder. In fact, physios can usually get all the necessary information needed from a physical assessment and a history of your pain or discomfort.
 
If it doesn’t repair, then how do I fix my shoulder?
If part of a rotator cuff is torn, then the job now is to strengthen the remaining fibres of that muscle, as well as the other muscles in the system. This is especially important when you have a painful shoulder, but as a preventative measure it is also a good idea to keep good strength in the system. Physio will be able to set and progress a good exercise program appropriate to you. 
 

Partial-Thickness Rotator Cuff Tear by Itself Does Not Cause Shoulder Pain or Muscle Weakness in Baseball Players - Teruhisa Mihata, Rei Morikura, Akihiko Hasegawa, Kunimoto Fukunishi, Takeshi Kawakami, Yukitaka Fujisawa, Mutsumi Ohue, Masashi Neo, 2019 (sagepub.com)