I don't really care who starts but I see no reason not to start Porzingis. He was the starter all season. What reason is there to change that? You can still manage Porzingis's minutes any way you want whether he starts or not.
To me, the bigger question is what line up do you close the game with. In the regular season, in the 4th quarter, Porzingis was on the floor with starters for 103 minutes and was +39. Horford was on the floor with starters for 79 minutes for -13. To me, it was pretty clear that Porzingis was the better option over Horford. That may not still be the case if Porzingis is not 100%.
The other scenario of course is that Horford and Porzingis were on the floor together in the 4th quarter for a total of 90 minutes and were +2 (again, 4th Qtr only). My guess is that most of that was not at the end of close games, but I don't know to sort for that.
So if we are in a close game, who do you put out there for the final 4 or 5 minutes? Horford? Porzingis? Both?
Having 6 starters is a nice problem to have.