Uneven bathtub drain hole, can't screw in new drain
Normally the drain unit in a tub or sink is pushed in from the top with plumbers putty under its top flange (Or some use a rubber gasket) and then it is pushed down into the tub opening. It is secured from the bottom side using a large size nut that screws onto the outside of the lower portion of the drain basket.

You will likely have a very difficult time trying to seal it up by turning and screwing the basket from the top.