The visitors capitalised on a calamitous mistake from goalkeeper Owen Evans as goals from Fleetwood’s new signing Will Davies and midfielder Elliot Bonds gave Peter Wild’s team a deserved three points.
After cruising to National League glory last season, Brennan has so often been a voice of support for his players, but Saturday’s rude awakening to life in the EFL saw his approach take a different turn.
“Both boxes – as simple as that. We defended terribly in our box, and we were all bark and no bite, and I’ve said that in a previous interview as well. We just didn’t have enough in the top end of the pitch,” he told the club website when asked what went wrong in the loss.
“Players playing their own game, doing their own thing, not following the tactical instructions. The last thing you can do against a team that wants to come here and sit in and be difficult and hard to play against is to give them a goal.
“Kamikaze, really, but that’s what happens when you don’t do things right, when you cut corners.”
At 0-0, Bees captain Danny Collinge thought he had opened the scoring but was penalised for being in an offside position.
The momentum then swung considerably in favour of the visitors, who soaked up Barnet pressure for most of the 90 minutes before leaving the Hive with all three points.
“The first goal was vital in this game, especially against the teams that are going to be so defensive. They’re gonna flick balls on, be difficult to beat, and plug the gaps,” said Brennan.
“We’ve got to stay in control of the ball, and we’ve got to stay in control of possession. When we have possession, they can’t hurt us.
“We start playing long and start doing our own thing, players doing their own thing – no player will do their own thing around me. They’ll be down the road; it’s as simple as that.
“I ain’t gonna pull no punches. The standards today are nowhere near to what we’ve set around here.”
And the Barnet boss continued to criticise the performance of his players on the first day of the campaign.
He said: “You have to dedicate everything to this business – everything. Their goalie made a brilliant save with his left hand at 1-1. Our goalie (Evans) makes a save, spills it to their lad, and they score a rebound.
“Their goalie’s made an unbelievable save with his left hand, so at the end of the day, when I’m watching a game of football, I’m not here to lie.
“It was a very disappointing performance, and the goals we gave away are just a shambles.”
Barnet will look to bounce back at Bromley – who drew 0-0 at Shrewsbury – in south London on Saturday (3pm).