The tree as it looks today:

{{gwi:637220}}

The tree in its settings - lime tree to its right and meyer improved to its left, beyond the lavender. Meyer is thriving and covered with tiny lemons; lime is fine but not giving any fruit.

{{gwi:637221}}

The leaf drop is a normal response to root loss. The tree does not have enough root mass to support the full canopy so its dropping part of it. Water this tree only sparingly. Overwatering will only promote root rot not root growth. Fertilizer will only further stress the tree. No fertilizer until spring.

You can help the tree recover by triming off 1/3 of the remaining canopy. This will allow the tree to concentrate on root growth instead of leaf/flower/fruit production. Keep the tree safe from frosts and it should bounce back.

As tempting as it is, don't baby the tree too much. You CAN kill it with kindness.
Andi

Trim the top with the same amount of root lost and cover the plant from the sun for 4 days to a week or until you see new growth coming out. This tree is in dire stress right now due to root lost and having lost its food source meaning the nutrients in the soil so what you want to do is lessen it as much as you can. Fertilizing it right now will add stress to the plant.

Dm, never over-water a plant, in the sense of overwatering.
Never feed an ailing plant either..
Since your citrus tree is stressed out, there is a product/hormome/50vitamins called Superthrive. Some people don't think it works, but I for one use it religiously. It works fantastic on stressed out plants..They may sell it at your local nursery, or you can order online. For stressed plants use 10 drops per gallon of water. Unless your tree is really in bad condition, (dying/dead) by using Superthrive you'll notice a difference almsot immediately. Toni

So:

I will hold off on fertilizer (as I thought).
I will not water it anymore than the other citruses.
I will trim a little more the canopy (had done some of this already, but should really do more to reflect root loss).

Bencelest: by covering the plant, did you mean partially (to prevent burns from direct sunlight) or entirely (to further promote root growth over leaf development?)?

Thanks everyone for the (consistent - that helps!) responses. I'll make sure to keep you posted on that patient!

Just so the leaves are protected from sunlight. You still want air circulation throughout.

DM:
When you transplanted the tree did you soaked the soil enough so the water was running everywhere? The purpose of flooding the tree the first time you water is to let the root hairs cling to the soil so there is no air gap between them so they can establish and suck in the nutrients again from the soil. If you have not done it do so now. But do it only once. Then you let it go. don't water again until the top soil is almost dry.
The way to do it is get a hose and run it in full force and dip the hose as deep as you can about a foot away from the trunk. When you see the water is coming out from the hole move it about a third distance away from the first hole. Do it once again a third distance away around the tree. You want to make sure no air bubbles is left under the soil so the remaining roots can once again get in contact with the soil and feed the remaining leaves.
You will have a leaf lose if the remaining roots can not feed all the leaves and that's the reason why you cut the branches in the first place. The branches that you cut will be replaced with perhaps double the number in 6 months. And more growth height if you follow what we tell you.

Bencelest: yes, did that. Not following your more scientific method though: I had cleaned the roots with a hose, cut clean the damaged ends then put it in the hole that I had previously filled with water, added the soil, compacted and then watered again.

Never do that again. Try not to disturb the roots as much as you can. But the damage is done so all you can do is hope.
When you transplant leave the soil intact with the roots as much as you can. It is the only way the plant will survive is continous flow of water and nutrients to the leaves.
Roots do all te dirty job but the leaves and flowers take all the credit.
You do away with the roots and the leaves will wilt and die. i.e. the cut up roses you give to your girl.
So next time don't do that. You may have damage the roots hairs also. So what if the roots look dirty when your neighbor gave it to you. What you should have done is wrapped the roots together with the reamining attached soil in a wet burlap and gently transferred it to the site and plant it as soon as you can so it will not dehydrate.
My method is not scientific but my own way. I just think that if I am the plant how can I survive the transfer the least stress possible.

Also if you have a grafting wax which I don't think you have, you could have paint the ends of the damaged roots (but first cut it cleaned and not tangled barks and root hairs at the end)so to stop further damage.