INDIANAPOLIS – Indianapolis Colts quarterback Anthony Richardson and center Ryan Kelly both left Sunday’s game after entering the NFL’s concussion protocol.

Richardson suffered his head injury in the first quarter, likely after taking a hit while scoring a touchdown on a 15-yard run. He left early in the second quarter after self-reporting possible concussion symptoms to the medical staff. Backup Gardner Minshew took over.

Kelly left in the second half with a head injury. He’s also in the league’s concussion protocol. Backup Wesley French played center in Kelly’s stead.

According to NFL policy, the protocol may be triggered by a teammate, coach, game official, booth spotters or team/unaffiliated medical personnel. A player first undergoes an examination in the sideline medical tent. If the exam is positive, inconclusive or suspicious for a concussion, the player goes to the locker room for further evaluation.

If the locker room examination returns a normal assessment, the player can return to the field. If something appears abnormal, the player stays in the locker room and can’t return to the game, as was the case for both Richardson and Kelly against the Texans.

Richardson and Kelly will go through a five-step process before being allowed to play again. Here’s a look at their journey back to the field.

Phase 1: Symptom limited activity

The player is prescribed rest, limiting, or if necessary, avoiding activities (both physical and cognitive) which increase or aggravate symptoms. Under athletic training staff supervision, limited stretching and balance training can be introduced, progressing to light aerobic exercise, all as tolerated.

Phase 2: Aerobic exercise

Under direct oversight of the team’s medical staff, the player should begin graduated cardiovascular exercise and may also engage in dynamic stretching and balance training. Neurocognitive and balance testing can be administered after completion of Phase 2 and the results should be interpreted as back to baseline.

Phase 3: Football-specific exercise

The player continues with supervised cardiovascular exercises that are increased and may mimic sport-specific activities, and supervised strength training is introduced. The player is allowed to practice with the team in sport-specific exercise for 30 minutes or less with ongoing and careful monitoring.

Phase 4: Club-based non-contact training drills

The player continues cardiovascular, strength and balance training, team-based sport-specific exercise, and participates in non-contact football activities (e.g. throwing, catching, running, and other position-specific activities). Neurocognitive and balance testing should be completed no later than the end of Phase 4 with the results interpreted as back to baseline.

Phase 5: Full football activity/clearance

Upon clearance by the club physician for full football activity involving contact, the player must be examined by the Independent Neurological Consultant (INC) assigned to his club. If the INC concurs with the club physician that the player’s concussion has resolved, he may participate in his club’s next practice or game.

Return timeline will vary

The time for a player’s return to the field depends on the individual player and the severity of the injury. Every player and every injury is different.

Tight end Drew Ogletree, for example, suffered a concussion in the season-opener against Jacksonville. He entered the concussion protocol and completed all five steps in less than a week. While he could’ve played Sunday, he was a healthy scratch for gameday.

Linebacker Shaq Leonard, on the other hand, suffered a concussion during joint practices with the Chicago Bears on Aug. 16. He cleared the protocol on Sept. 7—about three weeks later—making him available for the team’s first game of the season on Sept. 10.