Boxers box until the ref says they have suffered a concussion. But, the brain suffers concussion damage long before it becomes evident to the ref. The dazed concussion is a few levels worse than a minor concussion. Plus, the cumulative injury from multiple impacts (sub-concussive impacts) can easily cause a concussion without the outwardly visible sings that a ref might notice.

The current problem with contact sports is the athletes want to continue playing and often can hide the concussion.

Boxers often can back off and gone on the defense for a bit to get their wits back. This is so they can let their head clear. Just because their head cleared in 10 to 20 seconds does not mean they did not suffer a concussion. They do not need to be staggered by the blow for it to be a concussion.

btw, I have suffered at least 14 concussions. Only 4 were obvious to others.

Sometimes, the only symptom of the concussion is a strange taste in the mouth. This taste is often metallic like you are chewing aluminum foil. This is due to the taste processing neurons and/or connecting axons being traumatized.

Or, momentarily going cross-eyed, for just a few seconds.

There are other symptoms that will not be obvious to others and often disregarded by the person suffering the concussion.