If it is the kind of acer that has low and pendulous branches, the rabbits can eat all the leaves and lower branches off, leaving just the leafless stem, even if the trunk is so protected. I have used several defences: in one case I took a a piece of chickenwire about 10 ft by 3ft and make it into a circle to protect the whole tree. In another case I used canes to train the pendulous branches higher. In another case I moved the tree very close to the house so the rabbits wouldn't dare come so close.