Sounds to me like your acer is starving and also thirsty. Give it a thorough soaking asap and repeat every day or so till the root ball is thoroughly rehydrated. Give it some liquid seaweed or other fertiliser to perk it up. Keep it well watered.. Apart from anything else, this will make it easier to remove from its current pot.
If you plan on any root pruning, I would wait till the plant has lost its foliage in autumn and has gone dormant. Then take it out of its pot, scrape off as much of the worn out compost as possible and trim back some of the longer roots using sharp secateurs to make clean cuts. Then repot in a bigger pot if possible or in the same pot if you must and use best quality John Innes no 3 compost with a crock over the drainage holes. Firm it to the same depth of soil as before and make sure you leave at least an inch at the top of the pot to allow for generous watering or, better still, 2 inches so you can mulch it with expanded clay pellets, gravel, chipped slate or pebbles as you prefer. This will help with moisture retention and also keep down weeds.
Do the final watering and mulching once the pots is in its final place and check compost levels before you mulch. It will be much harder to more or correct after you've watered.
Every spring, give it a handful of slow release fertiliser such as blood fish and bone and keep it watered throughout the growing season. An occasional feed of liquid seaweed will keep it happier too.