We had a leak under our bathroom floor (1 story house, slab foundation). The plumber came in and jack hammered the bathroom removing 6 tiles (12" floor tiles). They fixed the leak, filled in the hole with cement and told us they don't repair the tile floor part. It does appear they made an attempt to smooth out the top of the cement. Thinking this would be an easy fix, we went out and got the tile. Just setting the tile atop the concrete, the tiles are already higher (like 1/8th or 1/16th of an inch but enough to notice) than the original tiles. Also, the tiles would wobble/ move back and forth like they floor was not flat any more. Its my understanding that the tile mortar will raise the tile even more so it seems that we some how need to lover the patch work they did.
Not being a home repair person, I have no idea how to lower or smooth out the concrete patch. It also seems like too small of a job to hire in someone to fix just 6 tiles.
You may have to manually chip away at the unlevel and irregular concete patch area using a masonry chisel. Another method is to use a grinder with an abrasive head (for grinding stone/masonry). I'd actually suggest using both.
Don't worry too much about taking too much material off, as this can be corrected (made up) with the thinset (adhesive), that you use,when you install the tile.