We don't know how long he will be sidelined. We don't know if he will be able to play Sunday. We don't even know for sure if he suffered a concussion.
What we do know is that brain health was one of the major storylines of the NFL's divisional playoff round. Mahomes was removed from the Chiefs' victory over the Cleveland Browns on Sunday after a hit at the end of a run left him staggering. A day earlier, Baltimore Ravens quarterback Lamar Jackson was ruled out of a loss to the Buffalo Bills after his head slammed to the ground.
What's in store for Mahomes this week? Let's take a closer look.
How could Mahomes be in the concussion protocol if he didn't suffer a concussion?
First off, we don't know one way or the other what Mahomes was diagnosed with. And importantly, it's not required for a player to have been immediately diagnosed with a concussion in order to be put in the protocol. All the Chiefs have confirmed is that he's in the protocol. On Monday, coach Andy Reid stopped short of saying that Mahomes had suffered a concussion.
Why would the NFL do this?
In 2018, the NFL adjusted its protocol to require in-game evaluations for "all players demonstrating gross motor instability (e.g., stumbling or falling to the ground when trying to stand) to determine the cause of the instability." That roughly fits what happened Sunday to Mahomes. The protocol goes on to say that if a doctor "determines the instability to be neurologically caused, the player is designated a 'No-Go' and may not return to play."
This change was in response to the scary injury suffered in Dec. 2017 by Houston Texans quarterback Tom Savage, who could be seen shaking on the ground after a hit but was allowed to remain in the game. He was later ruled out and diagnosed with a concussion. That adjustment allowed doctors to rule players out of games after examining them for these symptoms.
Mahomes, in fact, was ruled out even though he was running in the stadium tunnel after the injury, Reid said.
Does it matter if Mahomes actually suffered a concussion?
Of course. But whether he did or didn't, he must pass through the same five-step process to be cleared for a return.
What are those steps?
The first thing you should know is that prior to the season, every NFL player takes neurological and balancing tests when in a noninjured state to provide a "normal" score. Those results can later be used to help diagnose a concussion, and to determine when a player's neurological activity and balance has returned to its previous state following a brain injury. The five steps are:
Step 1: Based on symptoms, the player can engage in light stretching, balance training and eventually progress to light aerobic exercise.
Step 2: The player can graduate toward cardiovascular exercise and dynamic stretching, and then take neurological and balance tests. He can pass through this step once those test results match his baseline scores.
Step 3: The player can move toward a limited amount of football-specific exercise. That includes up to 30 minutes of practice time, under the supervision of an athletic trainer.
Step 4: Football activities can increase to noncontact drills such as throwing and running. Another set of tests must again show baseline results.
Step 5: This requires the team doctor to clear the player for contact. Once that happens, the player must be examined by an independent neurological consultant (INC). If the "INC" affirms the team doctor's decision, the player is cleared to practice full and play in the team's next game.
How long will all of that take?
The protocols intentionally carry no time requirements. They do not require a player to sit out a game, largely because the science of concussions show that brain injuries heal at unpredictable rates. Players could conceivably return to baseline quickly, without missing a game, or they could miss multiple games or even the remainder of a season.
So there is no data at all on that?
That's not entirely true. According to the NFL, using data from the 2015-19 seasons, the median length of time for quarterbacks to emerge from the concussion protocol is seven days.
The AFC Championship is a really important game. Don't players push through injuries all the time?
It is and they do, but the NFL built this protocol to ensure that doesn't happen with brain and neurological injuries. By requiring a return to baseline test results, the NFL's implicit policy is that a player with a brain or neurological injury can't return until he is fully healed. Football contact after only a partial recovery can exacerbate the injury.
Mahomes isn't supposed to be able to "will" himself back on the field or "suck it up." And the Chiefs aren't supposed to even have the opportunity to take the kind of calculated risk they do when they allow a player back on the field when he has, say, a mildly sprained knee.
How does the NFL prevent that?
The biggest distinction of the concussion protocol is that it requires an independent doctor to confirm the return. That doctor is not affiliated with the team or player but has been approved jointly by the league and the NFL Players Association. The final step of getting clearance from the independent doctor is intended as a failsafe for either the player or the team acting too aggressively.
On Monday, Reid told reporters: "There was a chance back in the day that Patrick comes back in [the game]. You saw him run up the tunnel. By the time he got to that point he was feeling pretty good. But there's a certain protocol you have to follow and that takes it out of the trainer's hand and the player's hand and the doctor's hand."
So when will we know more?
This will be a story for the entire week. It's possible we'll find out when (and if) Mahomes has moved on to Step 3, based on the Chiefs' injury participation report for practice. Otherwise, it's possible we won't know if Mahomes will be able to play until the weekend. The Bills-Chiefs game will kick off at 6:40 p.m. ET on Sunday.
Mahomes himself has suffered one reported concussion in his career, during the 2014 college season at Texas Tech. He returned to play in the team's next game, which was two weeks later because of a scheduled bye.