Kessler started all 58 games that he played in 24-25. He missed almost all of last season due to injury.
I went into nerd mode and compared their numbers per 36 minutes and they are nearly identical. Rudy (obviously bigger sample size) averages a point higher and half a rebound higher per 36. Kessler averages a full block per 36 higher, which really surprised me.
The point of this isn't to prove who is the better player. Rudy is. It's who would have been the better overall value for the franchise because of costs and draft picks not lost. Kessler would have been by far. In theory the team would have had comparable production at the position, much, much better production at the PF spot and the four picks.