I'm just recovering from a knee injury and I can add a little more on what I experienced from a diagnosis perspective.
My knee swelled up very quickly which apparently is an indicator of ligament damage. They test if you can resist them pushing against your leg from several angles, but it's difficult to test for damage to the ACL when the knee is swollen because of its location.
It needed an MRI and then analysis to determine that (luckily for me) I just had torn muscles around the knee. Although a scan will be done in a matter of days it can be a week or more for the radiologist to write the report, for a private patient anyway. For Saints this will obviously be accelerated.
Hopefully it's not serious for Fraser and it's just strained ligaments, but I fear the worst