Despite Johnson's recent bad fortune with injury, club general manager of football James Fantasia told afl.com.au the event was not an early retirement celebration for the 33-year-old.
"It was never set out to be a farewell. It was set out to be recognition, and I still subscribe to that," Fantasia said.
"He's having a run of injuries at the moment but who knows, that might actually prolong his longevity in the game.
"He's missed nearly half the season getting the other part of his body right.
"Obviously we'll make a decision at the end of the year as he will, but [retirement] is not a fait accompli."
For a footballer to receive a testimonial game AFL rules state the player must have served no less than 10 years and he must be nominated before the season starts.
While testimonials usually occur at the conclusion of a sporting career, Adelaide champion Andrew McLeod was honoured in a similar way last August before he entered his 16th season this year.
Fantasia said it was a unanimous board decision to honour Johnson while he was still playing, given his list of achievements.
"When we knew Johnno was eligible to have a testimonial, we just thought it would be fitting because it's almost the highest honour you can place on a player," he said.
"It's recognition of their career. It's not about money to give him a nest egg to finish off on; it's more about the club celebrating him and what he's given to the club and what he's achieved.
"We just thought it was a fitting year to do it because we're recognising our Hall of Fame and bringing everyone in the club back together again and celebrating the whole existence.
"We just thought it was a really good honour to place on him."
Last year Johnson passed Chris Grant - the last Bulldog to receive a testimonial - on the club's games record list and he is 143 games clear from Nathan Eagleton, the next most-capped current Bulldog.
Fantasia said those facts were enough to explain why the club captain should be honoured sooner rather than later.
"When you look back through history, to think you've still got the current games record holder playing, it doesn't happen very often," he said.
"If you look at the record, it's probably unlikely it will be broken in the future. In a sense, it makes him a living legend in his own right."
Johnson has also started a Twitter account, with Bulldog and football fans able to gain insight into his 17th season @BradJohnson6.
"He's been on it for a couple of days and he's already got over 700 followers. We thought it would be a nice way for people to get closer to him and go through the journey of his testimonial year," Fantasia said.
The day after the testimonial game the club will commemorate Johnson's extraordinary contribution to the Bulldogs since his 1994 debut at a luncheon.
Tickets for the testimonial lunch are expected to go on sale in late June.
