Bro:
what is the nature of the injury?? Not to scare you but STAY OFF that piece of shit!!!
The smith machine causes seriuos problems like pattern overload, repetative stress, muscle imbalance etc.
You can ruin your squat form (AND BODY) for good on that thing. Depending on your injury you are better with 1 leg squats, 1/4 squats with or without weight , physioball squats, chair squats etc.
You may need to isolate out weak or damaged muscles and then slowly integrate them into your squat form. The smith machine decreases balance, dynamic stabilization of the body and disrupts primal movment patterns.
Sorry but its the truth!!! I can give you reference's from Charles Poliquin, Paul Chek, Dr. Colgan etc The smith machine is good for one thing (besides an adjustable clothes rack) making money for orthopedists, chiropractors and physical therapists
Read these and you will understand:
http://www.chekinstitute.com/articles.cfm?select=27
http://www.mercola.com/2004/jul/28/fear_the_squat.htm
http://www.mercola.com/2004/jul/31/fear_the_squat.htm
S