The Cityzens struggled to create chances at home against the Sky Blues, registering just two shots on target.
Sydney came from Adelaide during the week after a cup final loss to the Reds and were clinical against City despite only four shots on goal.
While the home fans were visibly angry after the match, Brattan said he was unfazed about the result long-term.
The 28-year-old added the players weren’t affected by the outcome.
“The mood is fine, it’s our first loss and Melbourne Victory have lost two in a row,” Brattan said.
“We’re not panicking, but it’s not a great result. We didn’t play well, but it’s Round 3 and we’re pretty positive.
“Last week’s 1-1 draw to Central Coast did not play a part. That’s long gone and we should’ve got three points, but we didn’t.
“That’s football and we had a whole week to prepare for Sydney and we should’ve done better.
“I’m pretty frustrated, we didn’t create too many chances. I don’t think Bruno had a shot and every time he got the ball, there were four defenders around him and none of us near him.”
Brattan also gave credit to the Sky Blues and felt they could’ve been more tenacious going into the game.
“We showed them too much respect,” he said.
“They’re one of the best teams in the league and with the squad we’ve got, we need to play an attacking brand of football.
“Our final third is one of the best in the league and we need to give them better service and not give silly goals away.
“We had a lot of the ball, but they proved why they’ve been the best team in the last three years because they’re a great unit, hard to break down and ruthless in front of goal.”