The versatile Brennan was named at centre half-back for Sunday's clash against the competition leaders, but hours before selection said he did not expect to spend too much time in defence.
Brennan has been playing a back-up ruckman's role to Mitch Clark since long-term injuries to Jamie Charman and Matthew Leuenberger in the first month of the season.
But with the unavailability of defensive pillars Daniel Merrett (ankle) and Joel Patfull (jaw), the prospect of moving the utility back seemed a logical one.
However Brennan said he expected to continue his role in the middle and pinch-hit in the ruck against St Kilda hulks Michael Gardiner and Steven King.
"I don't think I'll be playing much at the defensive end because of our lack of ruckmen," he said.
"If I get given the task to go back I'll relish it but it's definitely not the first option. But if (Nick) Riewoldt and (Justin) Koschitzke start kicking a bag of goals I'll probably head back then.
"Our structures have worked well the last few weeks, so we don't really want to mess with those. I wouldn't think we'll change too much."
The Lions have named Tim Notting, Jason Roe and debutant Aaron Cornellius on an extended bench – all are capable of playing in the back six.
Which leaves Brennan's 192cm, 95kg frame up against the hulking Gardiner (199cm, 103kg) and King (201cm, 104kg).
"Jamie Charman is a few weeks away so until he gets back it works best for me to play in the ruck," he said.
"I'm getting more used to it and have changed a bit since the last time I played in the ruck a few years ago.
"I know I can't just run in and jump into the big blokes. There's just no way I can bang with bodies that are 20 and 30kg more than me.
"I have to run them around the ground and beat them with my second effort at the contests.''
Since Clark and Brennan have teamed up, the Lions have been thumped by Geelong, but then bounced back with a hat-trick of victories.
Brennan realised this was the first big test since the Geelong debacle, and said he'd been eyeing it off ever since.
"We're definitely not going in there thinking we can't win," he said.
"I've been looking forward to it for about two or three weeks to be honest. We came off against Geelong very disappointed; it was probably a wake-up call for us. The only way to get over that disappointment is to play another top team and this is our chance.
"It's going to be hard to win the hit-outs, but I've got to provide a contest so they don't just get their own way.
"That's always my focus, to win at ground level and provide a good contest there."
And with the reliable Merrett and Patfull missing, Brennan said the back six, with help from the ruckmen, would just have to do their best to stop the twin tower threats of Riewoldt and Koschitzke.
"They have been working well, but we're hoping our two big blokes (Jonathan Brown and Daniel Bradshaw) can kick more goals than them.
"We think we've got a couple of pretty handy forwards ourselves."
