Hi,
Sorry to hear about the torn ACL. I'm sure he will do great though. Dogs seem to recover much faster than humans do with that surgery.

As for the lump, you need to mention this to your veterinarian. It would be a good opportunity to remove it at the same time as the repair of the ACL. I'm not sure what it is; dogs are prone to all sorts of lumps and bumps (often benign) but it should be checked out.

Good luck with Barney,
drphilvegasvet