Zeller had a decent stat line. He is a nice pickup but I'm not sure He is a top 20 center.
The Other 2 were underacheivers. Both have to play well or it could be the end of the NBA dream.
All three have talent to be sure And Should be upgrades as you said. They have to perform.
Even if Zeller is not a "top 20 center" … he'll be an upgrade defensively at center over last year.
Last year, opposing centers _killed_ us.
I don't like to use PER for very many things, but it's useful for comparing identical roles.
Last year, here are the opposing center PER ratings against our various 'centers':
vs Sully: 18.9
vs Hump: 20.7
vs Olynyk: 24.7
vs Faverani: 20.5
vs Anthony: 18.3 (tiny sample size)
And meanwhile, for Cleveland
vs Zeller: 15.8
Zeller was a full-time center and his number is based on a large sample size.
So, even if Zeller isn't "Top 20" -- he'll still be a huge upgrade in that respect.
All numbers courtesy of 82games.com.