That is a bummer about your knee. Hopefully it gets well soon.
Don't feel silly just going to the gym just to do upper body exercises. Starting strength only has three exercises per workout anyway. You can still easily do two major compound moves per workout without legs.
Workout A
3x5 Bench Press
3x5 Chin-ups/Pull-ups weighted
Workout B
3x5 Press
3x5 Rows
That is a lot of working out and you could easily add other exercises to make three exercises: dumbell versions, curls, triceps extensions, hitting a heavy bag, etc.
Don't let a knee get in your way.