MRI shows that there is tearing of the pectoral muscle, but the tendon insertion into the humerus cannot be located. Apparently this is a relatively uncommon injury and an akward anatomical location to visualize under MRI.
Whether the tear is at the muscle-tendon interface versus the tendon-bone interface is important. When the muscle tears away from the tendon, there's nothing to be done surgically. When things scar down things tend to recover to an adequate level. But when the tendon tears away from the bone, surgically reattaching the tendon to the bone will allow for the best long term recovery.
Decisions decisions, surgery or no surgery?
Surgery is a freakin chore. The time and expense of surgery, plus two to four weeks of immobilization to allow for healing and then a very slow process of regaining range of motion and strength. Caring for children and work is out of the question during that period of immobilization, much less fun.
I really don't care if my chest looks a little uneven or I lose the big bench press. I should function just fine, even with 75% of my "normal" strength level. Right?
On the other hand, if the tendon is tore off the bone and I fail to have that surgically repaired, I'll end up with much less strength. The doctor says that there's really no way to tell without going in surgically, and it's up to me.