You can build big legs without squatting. Squats aren't mandatory, they are the best overall mass builder out of all the compound movements. I couldn't squat for a while due to lower back issues and my legs grew just fine.
What id suggest is obviously don't go to heavy, pre exhaust you quads with extentions before you go and squat, do hack squats. And last but not least whatever exercise you do make sure you knees stay in line with whatever way your toes are pointed and DO NOT let them buckle in or out, and push with your heels and not the balls of your feet, these two things will really help your knees from being injured or REinjured.