Lay ceramic tile over damaged green sheetrock around bathroom tub wall? The green paper peeled off.?

When I removed the old wall tile, the green paper peeled off leaving the brown cardboard like stuff exposed. If I tile right over that, will my tiles just fall off since there is no moisture barrier and mastic is wet? Plus the steam from the tub/shower. What's the solution? I hope I don't have to replace the sheetrock! Can I just cover it with drywall patch (mastic), let it dry then tile over that?