Fluid under kneecap

The boy has massive knee pain.  We took him to the doc in a box on the day before Thanksgiving because every place else was closed.  They did an xray and found fluid behind his kneecap.  Most likely cause?...torn meniscus, torn cartilage, or torn ACL.  I hate sports.  I hate my kid being hurt.  I hate watching him try to fight through the pain.  We have to see an orthopedist next week.  Praying it is something simple that won't need a surgical fix because he's my baby and never had surgery.  He is crushed because his season is likely over before it even began.  Hate seeing him sad.