RUCKMEN will be a victim of the new substitute rule and most clubs will make horrendous mistakes as they experiment with a three-man interchange bench, according to former Sydney Swans coach Paul Roos.

"The second ruckman has got to be able to play forward," Roos said. "(But) how are coaches going to handle it? It is probably a difficult question to answer because people are yet to figure out how it is going to work.

"I think some clubs will make some massive blues. But where it will end up is interesting.

"Do you have a second ruckman as your substitute? Well, that’s probably unlikely as well because (ruck) is a very specialised position. Is it an older player who’s a sub that you bring on? Is it a versatile player? Is it a key position player?

"It is a very, very difficult question to answer and I think each coach may have different ideas and I think that it will take to at least half way through the season (before they work it out)."

Roos said panic among ruckmen was for good reason and he believed the days of two beanpoles - who can do little else apart from ruck - were numbered.

"Clearly if you’ve only got three on the bench you are unlikely to have a second ruckman, so I think most people will say the second ruckman is a thing of the past, which is an unfortunate repercussion of (the rule)."

Roos said the obvious problem with playing one ruckman was a team could be stranded with a "part-timer" if the No.1 tall went down early in the game.

"Teams won’t want to sacrifice their number of rotations and the guys getting a rest…so it will be three versatile players and then one sub."

He said whether or not a ruckman was included on the bench as either a substitute, or in the interchange rotation, was "going to be a huge discussion by the match committee on a weekly basis".

"The other thing is if you don’t get an injury, what do you do? What time do you take a bloke off? If a bloke is having a shocker, do you take him off?

"If he is a full-back and he’s having a shocker, and the guy you sub is a 5 foot-11 half-forward flanker, what do you do? There are so many things that you’ve got to think about.