I herniated a lumbar disk in a sledding accident worthy of an episode of "World's Dumbest". I couldn't rack a 135 lb barbell 3 months later. No pulls, presses, squats - very depressing. going on three years now and I now squat dbw for two reps, but I just took it slow, did single leg body weight work until my back healed up, and once I could squat again, worked hard on keeping a neutral spine. I squat low bar and stop at or just below parallel because my back rounds dramatically going below parallel and my ass and left leg go numb. I go ATG with front squats with no rounding or pain. IMO, and based on that article I mentioned, I think that for most people, it is not possible to eliminate rounding if you squat below parallel with a low bar position. I don't think stretching helps - at least it did nothing for me.
Is your back strain a disk issue or a true muscle strain?