I would have given it some attention before planting. That would have been the time to address any rootball issues, including washing as much of the existing potting soil out of the rootball as possible, loosening and spreading the rootball, and if necessary, doing some root pruning.

But you can still do this. Just wait until late January or early February, then dig, make any rootball corrections necessary and replant (same as you would do if up-potting a containerized JM).

Other than that, just following proscribed planting methods (wide, shallow and unamended planting hole in well draining soil, exposed root flare, etc.) and watering as required next season should be all that is necessary. And maybe some protection from deer or rabbits if those are an issue for you :-)

I actually did most of those things before I transplanted the tree. I had to cut into the rootball, teased apart, loosened, and trimmed the roots. I pulled out some of the soil but I recall there were so many roots that there actually wasn't much potting soil there, aside from near the top. I'm wondering if I should fertilize in spring also?

Well then, it sounds like you have already attended to any pressing issues...... :-) As to fertilizing, while I always fertilize my containerized maples every spring, I have seldom found the need to do much with those in the ground. Usually just a good layer of mulch will supplement any required nutrients over time. JM's do not require much in the way of fertilization but a light dressing of Osmocote or HollyTone won't hurt.

Thanks!

Need for fertilization always depends on the specific conditions of the soil or potting medium involved in each instance. With any kind of plant. So without sampling the soil on your planting site and having it analyzed you will only be able to assume a particular approach is warranted or likely to be successful.