The loss of key defenders Daniel Merrett and Joel Patfull will be no excuse if an improving Brisbane Lions do not pass their biggest test of the AFL season, skipper Jonathan Brown says.

On paper, the Lions are roaring after cracking the top five following three straight wins.

However, they are vulnerable ahead of a game Brown believes will provide an accurate gauge on just how far they have come under new coach Michael Voss - unbeaten ladder leaders St Kilda at Etihad Stadium on Sunday.

Scans have revealed Merrett (ankle) and Patfull (broken jaw) will be sidelined for four weeks - a major blow for the Lions defence as they contemplate how to topple twin towers Nick Riewoldt and Justin Koschitzke.

Brown described Merrett and Patfull's loss as a "big loss" but said the Lions would not be backing down from the challenge.

Brown said the Lions were still smarting from their 93-point loss at Geelong three weeks ago when they "failed miserably" to test themselves against a top side.

Now another timely test looms for the Lions against the marching Saints - and Brown won't be tolerating anything but a committed team effort despite their injury woes.

"A lot is going to be made of the injuries no doubt this week, not having our best side - but it doesn't matter," Brown said in Brisbane on Tuesday.

"It doesn't matter what the personnel is.

"If we are playing finals footy and you have a couple of key injuries you can't just shut up shop.

"It is still a gauge for us on where we need to improve.

"It will be a great test."

Brown said the lessons learned from their Cats mauling ensured they had a "better mindset" against the Saints.

"We were very disappointing against Geelong - no doubt the guys have thought about that game

"But this is a new game. We have a better mindset, we are better prepared going in.

"It helps when the confidence is up. We are looking forward to testing ourselves."

The Saints may have made 2008's preliminary final but they have taken the AFL by surprise with their storming season start with the same personnel.

Brown said the Saints' shock transformation into an AFL powerhouse came down to "belief in one another" - something he could relate to strongly.

"It's amazing what it (self belief) can do," he said.

"In 2001 we were a pretty ordinary side at the start (of the season) yet we won 17 in a row with exactly the same players - it can come down to mind set.

"They (Saints) have their tails up. It's a formidable challenge but we have won three straight so we are up and going at the same time.

"We should have plenty of confidence in each other and our own game."

Meanwhile, the Lions have ruled out Troy Selwood for the Saints clash.

It will be the second straight game Selwood has missed since suffering concussion in a sickening head clash with Richmond's Alex Rance.

Selwood failed to pass a medical test on Tuesday.

Rance had to undergo surgery for fractures around his cheekbone and eye socket from the clash.

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