Christopher Ellis explains when stretching an injured tendon is safe. Gentle stretching is okay, but avoid aggressive pulling if the tendon fibers are frayed. Focus on active range-of-motion exercises instead to preserve mobility during early healing.
All right, what about stretching? I don't love stretching a tendon that's injured. You can if you keep it gentle, but, you know, if you picture like a, let's take a tendinosis for example. If you've got some fraying, you don't really want to tug on that too much. Again, it's probably pretty safe to do it gently, but also it doesn't do a whole lot. I'd much rather do active range of motion so you don't lose any range of motion. But in this early phase at least, you don't want to be tugging on something that's trying to repair.
