#1: Scott McDonald a big miss
Leaving out Millwall's most consistent performer was a decision that Ian Holloway deeply regretted in hindsight.
Having scored twice without Scott McDonald in their previous game, Holloway wanted the Lions to start against Brentford how they finished against Blackburn.
But that move backfired and Holloway acknowledged his mistake afterwards.
It was evident after 20 minutes that McDonald was being sorely missed.
In the first half especially, the Lions turned over the ball with unforgivable ease. Nobody could retain possession in the final third.
And not enough pressure was being applied on the visitors' back four – they were given far too much time to feed the ball into midfield unchallenge.
Millwall were chasing shadows for the first 45 minutes. Their closing down was half a yard slower than the Bees' crisp passing.
McDonald's absence was so noticeable because of the defending he does from the front.
The Australian sets the standard for the Lions' pressing, and without him the side lacked cohesion. In contrast, Brentford hunted the ball down in packs.
McDonald is also brilliant at getting hold of and keeping possession, either by jinking past a player of turning out of a tackle until support arrives.
It was little surprise that his introduction from the bench after 57 minutes almost turned the game in Millwall's favour.
"Scott had played every minute of every game and I wanted to give him a rest but boy oh boy did I rue the decision to leave him out," said Holloway afterwards.
Given that post-match admission, you'd think this won't be a mistake the Lions boss makes twice.
#2: Lions struggle to raise their game
Millwall are a good underdog but they really struggle in the games they are expected to win.
This has been a problem for a while now, stretching far beyond Holloway's nine-month tenure, and it's a huge Achilles heel.
Even when the Lions did beat crisis club Blackpool at the end of August, they did so by just the one goal and were hanging on for dear life at the end.
Leeds, Fulham, Nottingham Forest and Cardiff? No problem. Away to Wigan and Derby? You can rely on a dogged rearguard action and a clean sheet.
Big games against big clubs typically bring the best out of Millwall.
But Rotherham, Birmingham and now Brentford have all rolled up at The Den this season and landed fatal blows simply by scoring the first goal.
Even though they were beaten 3-1 by Watford at the turn of the month, the Lions still produced one of their better performances at Vicarage Road.
But on Saturday, Millwall started so sluggishly. The Bees were sharper, faster and hungrier.
Mark Warburton's side are new to the division and determined to prove they belong here. They did just that.
So the Lions must find a way of raising their game against every opponent – not just those with the biggest budgets.
#3: Lions boss has chance to name settled side
A combination of injuries, suspensions and three-games-a-week has caused significant disruption to Millwall's back four.
But looking at their fixture list between now and Boxing Day, Holloway may now be able to field a settled side.
The Lions boss has rejected the notion that he is "deliberately chopping and changing", saying that making regular alterations to his XI has been necessary to prevent injuries.
With Andy Wilkinson banned for three games and Scott Malone nursing a muscle strain, Holloway has had one hand tied behind his back.
He has chosen to keep things fresh at centre half, where Alan Dunne and Byron Webster have rotated with Danny Shittu and Mark Beevers.
Goalkeeper David Forde admitted after defeat to Brentford that the changes in personnel in front of him "hasn't particularly helped".
Thankfully, Millwall's next six games each come a week apart, so the Lions will have enough downtime between matches to be firing on all cylinders.
#4: Gueye deserves a second chance after error
A loose first touch from Magaye Gueye led to Brentford’s second goal but the Frenchman should not be cast aside after one mistake.
He looked lively when he came on against Blackburn in midweek and was rightly rewarded with a start. But like many others on the day, his performance was mixed.
And his final contribution was to gift the Bees possession in a dangerous area, allowing Alan Judge to tee up Andre Gray for their second goal.
He was substituted seconds after that lapse, although his withdrawal was already being planned before the goal went in.
Gueye was a £1million player four years ago. He's 24, his best years are still ahead of him and he has only just been deemed fit enough to come back into the fold.
He needs the chance to play his way into some form.
#5: Up and at 'em is effective
Holloway has hinted that Millwall could try a new formation after the international break.
"I might have to look at changing shape and playing a different way to give our boys some confidence," he said.
"When we went from back to front and put Brentford under pressure, we looked really good."
When the Lions commit bodies into the box and get the ball forward quickly, they have far more potency about them.
That was the case against Wolves, when they came back from 3-0 down to draw, and against Blackburn, when they repeated the feat with ten men.
The same gung-ho approach nearly paid off at the weekend, when they overturned another two-goal deficit to briefly get themselves back on level terms.
There is a fine line between a fast-paced direct approach and aimless hoofball, but this tactic did the trick at the end of last season and it may be the answer again.
The only problem? Millwall don't have Stefan Maierhofer anymore…