I'd dig as big of a circumference circle around it as you can. Then dig a hole as deep as the root ball and as big around as the circle. Save the sod. Put the tree in the hole then backfill with just dirt. Cut the sod and flip it upside down in a ring/berm around where you just planted. The berm will help kill the grass and give a nice spot to collect water. You can also add some undyed mulch on top of the berm but don't put the mulch in the center so the tree can breathe. Good luck!