After seven-straight wins and a relatively clean bill of health, the Lions' three-peat push received a massive shock at Marvel Stadium on Saturday night.
The Blues' 24.10 (154) was the second highest score Brisbane have conceded since Fagan's first season as coach back in 2017 when the Lions claimed the wooden spoon.
The only bigger total kicked on the Lions in the last nine seasons was the 166 by GWS in May when they slammed through a record-breaking 14 goals in the third quarter.
"We've just won seven games in a row and lost one game tonight," Fagan said.
"I'm not panicking at all, this happens in footy sometimes.
"It's not what we wanted tonight, it's not ideal, but it's happened and we've just got to respond now."
The Lions will be sweating on scans to veteran defender Ryan Lester (shoulder) and co-captain Hugh McCluggage (calf).
Luckless defender Keidean Coleman (concussion) and star young forward Logan Morris (corked leg) also finished the match on the bench.
"In the last quarter, we didn't have a player to bring back on the field, so we couldn't slow the game down," Fagan said.
"It was just difficult.
"The guys that were out there were tired.
"Carlton were running over the top of us.
"It was like your worst nightmare.
"But that's that's just the way it goes in footy sometimes."
The Lions were already missing Ty Gallop in defence, the young premiership player starting a three-game suspension for a serious misconduct act.
Brisbane opted to swing premiership forward Eric Hipwood into defence and recall Oscar Allen for his first game since breaking his foot in round seven.
The Lions' defence looked rattled, even three-time All-Australian Harris Andrews.
"Eric Hipwood doesn't play in the midfield," Fagan said.
"We got beaten in the midfield, we lost the supply battle, and the backs are under the pump.
"We had to try and come up with something ... we tried Eric there.
"Eric's played there in the past for us, but he practiced there this week.
"But it's a little hard when you don't play there every week."
Veterans Dayne Zorko and Darcy Gardiner are tracking to return to the backline for Friday night's blockbuster clash with Hawthorn.
The Lions will need to win all three of their remaining matches to be a chance of finishing inside the top-four.
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