Clubs will be able to replace an injured or concussed player under a new medical substitute rule that has been introduced less than two days before the start of the 2021 Toyota AFL Premiership season.

All clubs will be able use 23 players for each game instead of the regular 22, but that extra 23rd player will only be able to take the field after club doctors have assessed an injured or concussed player as 'medically unfit' to continue in the match.  

To be eligible for a medical substitution, the club doctor must decide that an injured player will be unable to play a game in the next 12 days.

Concussed players must already sit out a mandatory 12-day recovery period under new protocols introduced by the AFL for the 2021 season.

Club doctors must provide the AFL with a medical certificate on the first working day after the match as evidence the substituted player sustained the injury.

Any club found to be breaking the medical substitute rule can be sanctioned "for conduct unbecoming, or prejudicial to the interests or reputation of the AFL, or to bring the game of football into disrepute".

As a result, team selection will look a little different in 2021.

Clubs will name a 'normal' squad of 22 players and four emergencies the evening before the game, but they won't have to name the 23rd 'medical substitute' player until final teams are confirmed an hour before the first bounce. 

The new rule will only apply at AFL senior level – not in any state league or elite junior competitions.

If the 23rd 'medical substitute' player doesn't take the field, they will still have a senior game credited to their career tally. And on Grand Final day, an unused medical substitute player on the winning team will receive a premiership medal. 

The introduction of the rule brings the substitute player back into the AFL environment after a five-year absence. Between 2011-15, the interchange was reduced from four players to three but a substitute player was allowed to replace any other player at any time, for any reason. 

The AFL's executive general manager of football operations, Steve Hocking, said the new rule aimed to ensure the League continued to best protect the health and safety of all AFL players.

"The health and safety of each player in the game remains the priority and the introduction of a medical substitute ensures that player safety remains paramount in our game," Mr Hocking said.

"Over recent years we have continued to take action to strengthen match-day protocols and amend the Laws of the Game to improve safety.

"We continue to listen and learn and where we can reasonably make our game safer at any stage of the season, we will."