Changes this year, most notably one penalising players who slide into an opponent's legs, have resulted in significantly fewer ball-ups, even in games without the trial interchange cap.
"I've noticed that seems to have reduced congestion," the league's chief executive said on Thursday.
"It's a bit early to say after a couple of weeks of NAB Cup whether that's an effect (that will last).
"We'll get a better guide over the coming weeks.
"All of the different interpretations, minor adjustments to rules that the laws committee have come up with seem to have a positive effect on the game.
"But let's give it a few more weeks."
Demetriou said the planned interchange cap wasn't just aimed at combatting congestion, but also increasing fairness and reducing injury, although the authors of the league's own annual injury survey said recently there was no definitive evidence it would cut down the number of injuries.
And numerous coaches and players have forecast dire consequences if the cap is introduced at the current 80-per-game trial rate.
But Demetriou said similar doomsday scenarios relating to previous rule changes had proved unfounded.
"I heard a report last week that if it's less than 120 then players will be finished at 30," he said.
"It probably means that Robert Harvey would have played until he was 90 because he played when there were about 10 rotations and he played nearly 400 games.
"Whatever the number is - and we know what the number is at the moment but it will be reviewed - will be the number and people will work within that number."