Celtics are in BIG trouble right now, no pun intended
Most of the league has the same problem. Look at the two teams in the finals. OKC has 2 7 footers and everyone else that matters is a 6'-5" guard. IND has Turner and Siakam. They at least have Toppin as a 3rd sort of big. Thomas Bryant plays some center for them Turner is a legit center, Siakam more of a swing PF.
I wouldn't want to have any less size than IND. I think the OKC model is going prove hard for other teams to replicate if they try to copy them. I think you need a legit C on the floor most of the time. That means you need at least a starter and a solid bench center. You can get by with Tatum as the PF swing but you need a 3rd big that can play as a PF.
If we lose Horford and trade Porzingis, our two centers are Kornet (hopefully) and Queta. That 3rd big right now would be Tillman. This might be fine for the lost season but this is not nearly enough for a contending team. I would be OK with Kornet, Queta, and Tillman for 2025-26 but very disappointed if there was not significant improvement for 2026 and beyond.