McKenna told AFL.com.au this week Bock, 29, was on track to return to full fitness soon, but his goal of returning for Gold Coast's round one clash with St Kilda at Metricon Stadium next Saturday night had proved unrealistic.
"His goal was to play round one everything going well, and everything's gone well, he's just underestimated the work you have to do coming back off a broken leg," McKenna said.
"But he's tracking well and he'll be a very important player for us this year, which is really exciting."
Bock broke his leg after colliding with Fremantle defender Matt De Boer during a marking contest in round six last season, but in his three matches before that the 2008 All-Australian centre-half-back had a big impact in occasional stints as a key forward.
McKenna said the Suns had missed Bock more than they had thought they would last year and would relish the opportunity to unleash him as a swingman when he eventually returns.
"'Bocky' is so valuable to us.When we had the luxury of playing him up forward last year we had probably two of our better games at Metricon Stadium, and we know how well he can defend when he's down back," McKenna said.
"So with Steven May, Tommy Lynch, Sam Day and Rory Thompson coming on and Matty Warnock being able to hold up an end, all of a sudden he becomes a bit of a swingman for us.
"And Nathan Bock as a swingman becomes a very valuable commodity."
McKenna is content with the development of his young talls, saying key forward Day has started to show some good signs, taking 14 marks in his past two games after struggling early in the NAB Cup.
McKenna says 20-year-old Day, who played 22 games in his first two seasons, has had a tougher start to his AFL career than key forwards like Geelong's Tom Hawkins and Essendon's Michael Hurley before him.
"Everyone judges our tall forwards like they judge every other tall forward, but all the tall forwards at other clubs are part of a list that has the age profile to actually support them," McKenna says.
"I always take comfort when people talk about it taking four or five years in the competition for young key forwards to find their feet. Well, Sam Day is just going in to his third year, let's not forget that.
"And unlike Tom Hawkins at the start of his career at Geelong, Sam hasn't had Cameron Mooney sitting alongside him, or Steve Johnson and those types nurturing him and allowing him to find his feet.
"It's the same for all our key forwards, they're thrown in at the deep end and expected to perform."
But McKenna says he is confident this baptism of fire will ultimately fast-track the development of Day, Lynch, 20, and Charlie Dixon, 22.
"I don't think it will take our boys five years to come on and that's why this year we'll start to see a bit of a turning of the tide," McKenna says.
"More importantly, in their fourth seasons next year I think that's when the development will really be there."
Nathan Bock is a defender/forward in NAB AFL Fantasy. He averaged 61 points in 2012. Register your team at our AFL Fantasy Hub.
Nick Bowen is a reporter with AFL.com.au. Follow him on Twitter: @AFL_Nick
