There are multiple different tests you can do to assess the stability of a knee ligament, whether on the field or a clinic or training room. None are 100% sensitive or specific though, the ACL ones generally range from 80% to 95% depending on what articles you read, and all the tests are less good in acute knees soon after an injury. Its also hard to test stability on a huge guy with massive leg muscles like Campbell.
The staff doing them will have a decent idea based on the mechanism of what happened, post injury symptoms and tests; however you are right the MRI is the gold standard to show exactly what the damage is throughout the knee and guide the next steps.
For what its worth (which isn't much based off a 3 sec clip) the side on video on this thread seems to show a valgus force, which is more commonly associated with MCL injury, and the foot was only momentarily stuck in the ground, but he has very big dudes off their feet crashing into the knee at speed. The question is what grade, and is the meniscus and/or ACL damaged as well, which may or may not be the case. Only the MRI will reveal that.
Here's hoping for a best case grade 1 or 2 MCL and back for the playoffs (or just before). Worse case unholy triad including ACL and IR.