If you tear some paper here or there it can be repaired with a skim coat of drywall mud. If the drywall is falling apart or has holes or crushed in areas
in a word save your time and REPLACE!!
If you haven't removed the tile as of
yet get a 5 in 1 tool they are excellent for tile removal. Get behind the tile
with the flat side of the tool blade.
Carefully remove so as not to damage
the drywall as little as possible. If you
tear some paper or make minor holes or
damage skim coat with drywall scrape or
sponge sand the drywall compound, dust
off and vacuum your mess and get ready to tile.