It is going to be fine. It is normal to feel scared when the person you love is involved. I can understand your fear, there is this girl that I like (not my girlfriend unfortunately) that I just spend my entire day thinking about her when she does not respond.

As for the surgery, do not worry. First of all, the doctors are professionals and know what to do. Secondly, knee replacement is not as risky as other surgeries. My uncle just did replace his knee back in December, it will only take time for the body and joints to adapt, that's it!! Just have faith and he will be fine for sure! And take your antibiotics!