I can understand your worry.

You should be fine. It’s not a done thing to downgrade other pax for VIPs or PPS at SQ.

Check in online as soon as it opens.

However, it is hard not to ignore your concern, seeing someone random on the internet is telling you not to worry.

I would therefore suggest you call SQ and ask them to put a note in your booking that you need to travel in your booked biz cabin due to knee injury (just be prepared they might ask if you want a wheel chair…I presume you don’t from your post 😉 ).

That way you can hopefully set your mind at ease as you can feel you have done everything possible to ensure a smooth journey.