The general rule of thumb for tile setting is that that replacement area should be at least the depth of the tile itself + 1/4" for the mastic. You will need a small rotary hammer with a bushing tool and a 3" chisel (those are designed for removing thinset), first "bushing" the high spots then scraping it down relatively smooth necessary depth. Are you confident the plumber did a decent job with the patch? Did he dowel the concrete at all? Also, do you have the properly grooved trowel for applying the mastic with?