Just my insight here:
I had a very similar injury 14 months ago where I herniated a disc. Just 4 months ago I started squating and doing DL's again. My problem was weak abs. I never had problems/pain in my rehab doing ab work and I was able to build my abs up.
An ab exercise you can try is this :
Lay flat on your back. Put your hands under your butt, one hand over the other. Proceed to lift you ankles a few inches off the ground. You can do repititions and you can hold at the top position. You might feel a little strain in either your abs or lower back where the injury is. If you feel pain then don't continue with the exercise.
You also might want to contact you PT with inquiries on different types of ab exercises. Also just look into all different types of ab exercises and do a little trial and error, find out what feels good and throw out what doesn't.
In an above post it was mentioned that one you have a pretty serious lower back injury it can be re-occuring. This happened to me twice through trying to rush my way back into the "good" lifts. I found the key was patience, strengthening core muscles and starting slow. Some examples that helped me. Back extensions on a GHR bench and ab work. Pretty soon I was able to front squats (I researched and found that front squats put less pressure on your lower back than back squats...and they work the abs pretty nice) with just the bar with no pain. After 2 months I was easily front squating 225 and then I decided it was time to start slow with the deadlifts and the power lifting squats.
Just some insight