The Tigers were overrun by Carlton in the final term after leading by 14-points at the last change.
Both teams appeared tired in the second half with a fitter, more experienced Carlton side led by Chris Judd powering home in the last quarter.
Hardwick admitted with one less player on the bench, his young side just didn't have the legs to finish the game out.
"Look, I thought it was okay but it [the substitute rule] is going to catch some sides out," Hardwick said after the match.
"You saw for our guys, they're incredibly young, they haven't had a lot of pre-seasons under their belt and it certainly had the desired effect on us in the last quarter.
"You look at Carlton's guys, they’ve had seven or eight pre-seasons [while the] majority of ours have had two.
"It is going to catch a few sides unaware, especially the younger sides I'd say, but it's had the desired effect, it's what the AFL wants, so it's going to work."
The game clearly slowed down after half-time with players more spread out over the ground than usual, while defensive zones were substituted for one-on-one contests.
"I think both sides looked exhausted by the end of it," Hardwick said.
"I don't know how it appeared on the ground but up [in the coaches box] the defensive formations behind the ball just really fell away, which was what [the AFL] was probably trying to do [with the new rule]."
The Tigers also lost a runner after half-time with midfielder Nathan Foley succumbing to a gastro bug that had plagued him all week.
Hardwick had planned to substitute on an extra runner with fresh legs in Daniel Connors in the second half, but hadn't expected Foley to be the one he needed to replace.
He admitted he made a mistake with his use of the substitute.
"I made a blue to be perfectly honest," Hardwick said.
"Axel Foley had gastro all week and I sort of half forced his hand to play, which of course cost us in the end.
"He just couldn't go on and had vomiting and diarrhea.
"So that was my blue. Dan has obviously had an interrupted pre-season also and we just tried to get some experience on the field after half-time."