need help-5x5 squat sub?

[MARK67]

hey guys,hope you can help, my bb has been sidelined again. ive managed to really mess my lumbars up,again,last time squatting 500 for 3 reps, did deads the day before,the pain was excruciating!it was a short lived record for me,this time,approx. 3 mos. later done in by poor dlift form. the question i have is i want to start the 5x5 mc program, but i cant hardly squat(225 for 8) it says no subs , any suggestions ,should i still try the 5x5,with less than max lbs on my squat,should it(the 5x5)still work? thanks for advice.