After holding their own in the first quarter at Etihad Stadium, the Suns were overpowered by St Kilda's influential veterans, falling to their eighth loss in 10 games.
In the final three quarters the Suns had 85 less possessions and 24 fewer inside 50s than their opponents, and McKenna conceded his team had been out-worked.
"After quarter-time, that was probably our stinker for the year I reckon," he said.
"We haven't had a game like that all season I reckon, and I think that's shown us where we've moved to.
"I'm not saying we took a step backwards, that happens … we've certainly moved a great deal forward this year."
After notching five of their seven wins in the first half of the season, McKenna said he had recognised some "creaks and groans" in his team in recent weeks.
He said personnel issues were starting to take a toll, with Greg Broughton (Achilles tendon), Andrew Boston (sternum) and Daniel Gorringe (groin) all injured on Sunday.
"I reckon since round 14, every player we've got back we've lost another two along the way," McKenna said.
"We'll lose another couple out of this game today."
Rookie Leigh Osborne made his debut on Sunday and "will go into a pre-season knowing what it takes to play at this level".
Jacob Gilbee also came in for his second game this season, finishing with 12 possessions and three inside-50s.
While pleased to unveil another debutant, McKenna said the Suns were past the phase of gifting games to players to expose them to senior football.
"They have their opportunities and you either grab them or you let them slip," he said.
"Some made it, some didn't, and some there's still question marks over."
Twitter: @AFL_Nathan
