kornet's problem in terms of the match up with dallas is that he does not spread the floor with outside shooting, among other things. this means that doncic can play off of him and clog up the middle. this lets doncic rest up a bit instead of playing defense near the 3 point line.
if the celtics put kornet in when doncic is on the bench, the celtics will be all right. doncic has averaged 40 minutes a game in the first two games. this means kornet can be in the game when doncic is not.
so kornet should be able to play 8 to 12 minutes a game. horford can play the rest of the minutes for a few games.
but i also think that the celtics are the better team even without porzingas.
but i also also think that porzingas will be able to play 15 to 20 minutes, so kornet is a non-issue.
EDIT
here is a clip of tilman that is interesting back in march against dallas.
https://twitter.com/Timi_093/status/1763924788727181700this is what we could hope for in 4-6 minutes from tilman if porzingas is unable to go. tilman did a nice job on Luka.