They are objecting Nagarjuna not to do the role. They say that earlier Nag performed as Anamaya and Ramadasu, who were said to be devotes of God, where as Sai baba is considered as the true spirit of God, so they said that Nagarjuna has no right to play this role.
It is not the first time that these type issues raised opposite Nag, Earlier in the time of Anamaya, same sought of issues were raised but Nag kept all these controversies aside and delivered his mettle in that movie. Hope he does that same this time too.