Well if he doesn't stay on the team then, why wouldn't you go the trade route first. Why are people even talking about stretching if he's waived. You may as well look for a trade to get his whole contract off the books and try and keep Simmons and/or Green instead. Is the chances of keeping Simmons and Green possible if TP is traded instead of stretched or does it make no difference? Sorry if this has already been asked and/or answered in the thread before.
If you can re-sign him in the same season, though, then maybe stretching is worth looking into, but that would only be worth doing if the Spurs think he can still be a helpful player which no-one is going to know coming off of that injury, so again, may as well look at trading him first if you need that cap space for a big free agent signing.