[KCMike]

Well I'll throw out an idea though I'm novice at veneer work.
By the looks of the veneer in the picture, its pretty thin where you sanded, you might be better off cutting off the whole sanded area. If you have a piece of veneer big enough that will match.
Using a razor knife cut at an angle across the grain. I would go along the bottom side and cut out the piece you already put in then do similar toward the top.
Use a heat gun to carefully loosen the glue and with putty knife and lift out the section you will replace.

Now if the veneer is still thick enough. you could just widen the area where veneer was sanded away and try to patch in a new piece.
Others here on the phorum may have better ways of doing it. Just an idea. Hope this helps.