McKenna said while he expected more of his team, he was satisfied with the effort and thought they would bounce back quickly next week against the Western Bulldogs.
With 12 debutants in his team, the first-time coach said it was a necessary step.
"You'd have to say in the club's journey it was always going to be a tough ask but it was a step you had to take," he said.
"You live in your home, in your castle, and you have to go out and eventually see the land, you've got to take that first step. I think in most journeys the first step is the toughest.
"Yeah, I probably expected better, but you've got to weight that up. Even the leaders themselves, the mature players, got swarmed as well, largely due to Carlton's pressure."
Carlton jumped to a 50-point lead after the first quarter and McKenna said the huge build-up may have got to some of his younger players.
"(There was a) Lot of jittery possession, lot of one handed stuff, lot of handball, just very basic stuff. It's almost like we're looking at the under 18s for Gold Coast out there and I think it was just the pressure and nerves of our first game we had to get out of the way," he said.
"It's different playing in the NAB Cup to round two that's for sure.
"It was massive for these young blokes but that's good. If we make a final this year at least we know we've been through this (build up).
"It looked bad on the scoreboard but throughout the course of the game there was some exciting parts where the boys were able to play our style of football and it looked good and we know over time it's going to get better and better and better."
McKenna said in his three years of coaching the Suns at TAC and then VFL level, the players had shown strong character and he was convinced they would bounce back.
He said they were hurt by the performance but the result was not through lack of effort.
"For all these boys it's been such a long time coming and they can now get into the groove and it'll be a nasty shock against the Doggies."
The views in this article are those of the author and not necessarily those of the AFL or its clubs
