Symptoms of limp, droopy leaves can indicate either too much or too little water or it can also indicate transplant shock.  You should check the moisture level of the soil by inserting a screw driver or a moisture meter probe (available at any garden center) to see if your soil is too wet or too dry.   Working up the soil gently around the surface of the tree will be helpful to ensure adequate soil oxygen to the plant.  You should wait to mulch the tree - once you have assured proper irrigation, continue to monitor the tree as it should recover from the planting shock over a  period of at week to 10 days.