Repairing loose tiles on shower wall - Tile Advice

[GPag]

Hi,
I need to repair the tiles shown in the photo. This is a shower stall where the grout failed and water got behind the tiles. The wall board was wet but not soggy. This is on a first floor; there is no sign of water leaking in the basement. The surrounding tiles are solid. I will be removing the grout in the entire shower stall and regrouting but what is the best way to repair the loose tiles? Do I need to cut the area out back to the studs?
Thanks

 

[J K Tiling Cornwall]

Depending on how easy those tiles come off I'd either take the board back to the studs, should be one in either corner or at least back to dry, solid board. Could fit some treated 2"X1" for support. Could break the board back below the tray and slot a new piece in behind, I'd seal it using a hybrid sealant and adhesive in one. Go for a cement backerboard that won't break down.