lower tile repair in shower

The shower walllower row of tiles had pushed out. I usued a little multi tool and removed the entire lover row of tile intact. It appears that the backer board was almost vertically flush with the up turned lip of the bottom pan one side but a good 1/2" or more set back from the edge on the other, that side filled with? Thinset and feathered to the other side. The push out of course was the result of the swelling of the back board the bottom two inches of the "fill side" being crumbled and rotten up about 3 of the 4 inches of the tile height.

I have thinset some tile over concrete before but that is about the total of my experience. Should I be able to get this right or call a pro. (hard to get a real pro for such a small job.)

Call a pro and watch the job grow when he tells you it must be done correctly and what you are suggesting isn't the answer to your problem.