NORTH Melbourne coach Brad Scott has slammed the AFL's decision to introduce a substitute for the 2011 home and away season, and says the new interchange rules will have to be altered at the end of the season.

The Kangaroos trialled having three interchange players and a number of substitutes on the bench during their 13-point NAB Challenge victory over the Western Bulldogs in Ballarat.

But Scott was far from happy with the outcome.

"What today highlighted is the disaster that is the substitution rule," he said.

"I'm staggered that we spend our time in the coaches box now managing and mitigating fatigue and injury."

Scott was moved to voice his concern after four of his men - Matt Campbell (hamstring strain), Sam Wright (concussion), Brady Rawlings (hamstring tightness) and Aaron Edwards (corked thigh) - were injured during the match.

A couple of those players were hurt after the North Melbourne coach had elected to use his quota of substitutions during the half-time break.

"In the second half we had players come off and we had to leave injured players on the ground because we've used our substitution and can't get him back" Scott said.

"All coaches voiced their opposition to this rule, (but) it's in play.

"This rule was supposed to be introduced under the guise of player health and safety. Well, we're going to see players injured having to stay out on the ground. Otherwise you'll be playing with 17 men.

"It's really, really frustrating. If this had been a home and away game today, we had four players who weren't able to come back on and we'd substituted someone off already.

"We've got a fit player who's been substituted off, we have injuries, and we can't bring him back. How that promotes player health and safety, I'm not sure."

"Instead of coaching tactically, you're wondering 'how can we keep players fresh?'.

"I'm not downplaying the role of senior coaches, but I thought we were employed to do more than that."

Scott is convinced the interchange rules will need an overhaul before the 2012 season.

"I would say we won't have one sub next year … we'll have more subs," he said.

"And that's the fairest way, isn't it? If it's three and a sub, why not have three and three subs. Then you're not losing a game because you get two injuries.

"You'd hate to lose a game of AFL footy because you'd subbed someone off, then got an injury and were one down."