The sub is back.

The AFL has announced clubs will be able to use a medical substitution this season.

Each club will now name 23 players in their side each week.

The substitute must come from the list of three emergencies.

While initially slated as a concussion substitute, the league has expanded the rule to cover any player who will be unable to play a game in the next 12 days due to injury.

Clubs will be required to provide the AFL with medical certificates to prevent the rule being exploited.

The news comes just a day out from the season opener.