I don't think Danny or Doc are expecting Darko to transform into anything all that much different than what he has been. One thing that I believe you can expect from Darko based on his track record is durability (same for Collins).
If you consider what issues the Celtics have had with durability of centers (Perk, Shaq, Jerm,...) then you can see why durability is very attractive to them. Over the last say 4-5 years we have done really well whenever we had even a decent center that was healthy and on the court (allowing KG to play PF).
Even during the Shaq run (when he was healthy) where we played so well, Shaq was not playing all that great. Going further back, I think it is clear that Perk was not all that great either but until he got hurt, he was plenty good enough.
I am hopeful that Darko and Collins can stay healthy and I don't think they need to be all that good and can still make a big impact.
Just consider a scenario where Darko learns the defense and plays well enough to not give up offensive rebounds or turnovers, doesn't foul out, and finishes when he is left open. That would allow us to put out a line up of Rondo, Pierce, Green, KG, and Darko. How does anyone match up with that? That will be a good team.