I am surprised that there was just drywall behind the tile. I would put an excess door on the back wall behind the plumbing for future access to repair if needed ---Hopefully there is a convenient wall behind the one you took down. Then instead of replacing drywall --I would install wonderboard or blueboard (consult your homebuilding supply store or Home Depot) it is a concrete type wall board made for installing tile onto for applications around water. It is more durable against water damage and the tile will adhere much better ..you do need studs to nail the edges securely to mainly enuf nailed locations so the wall board will hold the wight of the tile and mastic ...you want to make sure at least the the seam is cover by solid pieces of tile when retiling. Consult Home Depot for the type of mastic to use also...tell them where it will be used ...for example in the shower. Hope this helps. let us know how it turns out.