Ive had ligament reconstruction and bone spur now getting foot drop as not used foot 4 3 months what is this and is it serious?

Knee?: If your surgery was at the knee, you're most likely developing a problem with your peroneal nerve. Coming on months after a surgery means it wasn't damaged in surgery, but by some other process occurring afterwards. Usually it is something pressing on the nerve, like a cast, splint, or even crossing your legs a lot. Look up where the nerve runs, and then see if anything presses in that area.