Essendon coach Matthew Knights has called for the introduction of a fourth AFL umpire to keep a watch on off-the-ball incidents.

Knights said on Tuesday an extra umpire could oversee players in congested areas and help prevent illegal incidents.

He was commenting after the series of incidents involving St Kilda tagger Steven Baker and Geelong star Steve Johnson during the game at the MCG on Friday night.

"I think a fourth umpire to look at congestion of 20 men around the ball is important for the modern game ... particularly around those scrimmage areas," Knights said.

"It then may also allow the umpires that are in their zones, in their 50s, to keep an eye on their zone, being their forward lines at both ends or back lines.

"So it allows the central umpire to control the game, the secondary around the stoppages to watch the stoppages, and then the third and fourth umpires to look in the 50m zones.

"I think that makes sense."

Knights says players are still likely to act outside the laws of the game but having an extra umpire would help keep an overall watch over groups of players.

"It may not prevent it because it's player choice at the end of the day with their actions and behaviours.

"But I think an extra pair of eyes particularly around the stoppages - an extra pair of eyes around a congestion of 18 to 20 players ... that would be a preferred option from my perspective".

Knights, however, indicated that taggers could sometimes be unfairly targeted.

"There's different types of taggers, and we get defenders playing on forwards every week, so we've got to be very careful (we don't) put taggers into a box of being the villains or being the ones always committing these obscene crimes.

"I mean, come on, it's a very tribal game, it's a physical game and there's a lot of physical clashes with all players every week so I don't think we should be laying prejudice against the taggers.

"Everyone's got a role in football.

"Not everyone can be the stars, not everyone can take the high marks and there's a role for different people and different personalities and different body shapes and sizes in our great game.

"As long as they're living within the rules and they're legal. I think you can still be legal and still have a very physical game of AFL football," he said.

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