Sorry to hear about your injury, but glad to hear you are healing. I would agree with your therapist that strengthening your upper back is key: the connective tissues need some muscular support now to take the stress of them while they heal.
Yin Yoga is great for stressing the connective tissues and poses like
Anahatasana can work the area you are concerned about, but I do wonder if Yin Yoga is what you need right now. Yin Yoga will help stress the upper back connective tissues, but as your therapist says, you need to strengthen the muscles there. I would suggest you try some gentle yang yoga for this area, at least until you feel you are strong there and there is no pain. Then you can add back some yin poses to start working the deeper tissues.
Some yang yoga poses you can consider trying includes Eagle Arms (often done in Garudasana which is a standing balance pose, but you can do Eagle Arms at anytime: even while reading your computer). This will stress the upper back muscles via abduction of the scapula. Then you can try adduction via Yoga Mudra, which is clasping the hands behind your back, with fingers interlaced and then moving the hands as far away from you as you can and lifting the arms up and down. Again, you can do this anywhere and anytime.
Other poses like Down Dog, Dolpin, Plank pose will also help to strengthen the upper back: try them out but don't be aggressive with them. Deep seated twists can also strengthen the upper back muscles.
In a couple of days I will be releasing the next YinSights Newsletter: in this edition I will be describing some ways to do Yin Yoga for the upper body - if you haven't signed up for the Newsletter you can do so
here. You might find some suggestions there work for you, once you have strengthened your upper back muscularly.
Cheers
Bernie