More water. It's still living on the pot roots system..and the ground is wicking away the water. You're in a dry hot climate..water -build a well around it,and keep it mulched on planting is the no.1 priority.

Your are correct also that Mangoes sold often don't have root balls knitted together yet and I know some people have had the whole ball of soil fall apart when slid out of the pot. THAT is a major setback.

So when will I have to fertilize it then?

I would wait until it starts to flush new growth..by then its safe. You could help it now with liquid iron..not a fertilizer but a trace element they need. Darken up the foliage.

Vitamin B-1 does help with transplant shock... I should have mentioned that sooner. I've seen it work myself..on plant used,fine,not used on the same type..it died. So,I'm convinced.

But I’m not sure Although I have sandy soil the water does not seem to drain very fast even though you can literally see the sand in my soil yet the drainage Is not very good

And how much liquid Iron should I use?