Krstic and Murphy are no defensive studs, but once they learn the system and KG gets to them, they will look significantly better than they have.
By the time they are ready, they should only be facing backups anyway, and there aren't too many skilled backup bigs.
We won't be as good as we were with Perk (unless JO returns to near full strength - because I think he could actually be better than Perk on both ends), but our offense should make up for it. Krstic looks like he will get those open FT-line jumpers all day. Murphy will get looks and once he shakes the rust he will knock them down.
As long as the team is healthy, we are a better team than we were in January.