Speaking of Turner, I also believe that losing him has a great impact as well. Losing Hali for a season because of injury is already a bad thing, but losing your main center and has been in your team for a decade, and going to your "rival" team makes it even worse. On the flip side, they signed Isaiah Jackson to a 3-year $21M deal to fill Turner's shoes and he will be their starting center next season, but I don't know if he can do it TBH. As for Siakam, I expect him to be their number one guy next season in terms of offense, but the Pacers are pretty much balanced as a team so many will help him still.
One thing’s for sure, he doesn't have the outside shooting that Turner had. But maybe that’s not such a bad thing, since he can focus more on defense. What really exposed Indiana in the finals was their lack of interior defense. OKC wasn’t even that hot from the outside, but they played smart using two bigs and attacking the paint hard. That’s something the Pacers need to learn from. They have to adjust and exploit mismatches better.
On paper, Turner’s numbers look good, but this change was bound to happen. When a player has already made up his mind to leave, even a high contract won’t stop it. So might as well accept the transition and build around what they really need now.