Second opinion on an injury is never a good sign. Usually that means the original diagnosis was bad and they are making sure that is verified
Given how he was hit (I've only seen pictures, no one has been able to give me a video), I am concerned about possibly some PCL or MCL involvement, partial tear most likely. I wouldn't expect meniscus, but again all I've seen is pictures.
Ligament damage would make sense as you usually don't have pain except immediately at the time of injury. Many players return and play on a torn ACL, PCL, etc without any pain (the ligament is ruptured or significantly stretched and will only be painful if torn further if only partially torn).
It is possible a second opinion is to determine if surgery is warranted or a brief rest will be enough to allow the ligaments to heal. That is a very likely scenario, but again I have as much details as you do on what is actually wrong. My inside info does not include the Broncos. My prediction would be partial ligamentous tear based on how he was hit, how he reacted and that he was able to return, but now is seeing a second opinion
Edit: I thought this was his left knee based on pictures, I do believe ACL is a major concern here as it is the right knee he was struck on