Imran Tahir deserves a spot in the squad because of his bowling and they can shuffle according to the ground conditions, the local players needs to be replaced by young talents.
I would rate Tahir as one of the best spinners available in the IPL. But unfortunately for him, he is in the wrong team. CSK will be unable to accomodate Tahir, unless one of their regular overseas players get injured. Right now all four of them (Curran, du Plessis, Bravo and Moeen Ali) are very integral members of the squad and the management can't think about dropping any one of them. Mitchell Santner is also there in the CSK squad and he is a very good all rounder. But right now he has zero chance of getting selected.