Having had 4 ACL tears, and sprains and partial tears on top of that, they medical usually is enough to determine if the joint is torn. The MRI is just to confirm what the previous testing determined. Sometimes there is so much swelling, the MRI cannot determine the extent of the damage, and have to wait.
On occasion, the Pre-testing is not clear because the looseness of the joint isn't so far out of range. The fact that it isn't swollen yet is a good sign for sure, but doesn't mean it's definitely good news either.
I am cautiously optimistic, but it could go both ways, as I have experienced all different situations myself.