IF it's weak and that's a big if... but then again it depends on what sports you played and how your body adapts to the exercises.... you may need internal rotation.
Otherwise, You don't.
In most people external rotators are waaaaaay weaker than internal even in the same cases. I would prefer to bring up the imbalance first, then focus on evening out after a better strength ratio is established.
This can all be done with some specific tests though... you should only be able to internally rotated maybe about 3:2 ratio of what you can externally rotate. So 15 lbs internal to about 10 lbs external. I've seen cases where it's 25 external and about 5 internal so... obviously you don't need internal work at least yet if that's the case.
OP -- also forgot to say that don't do anything that aggravates it. If any exercises aggravate it then lay off them for now.
REST + fish oil + massage is key to getting the pain and inflammation down.