October began with a blow as Byron Webster was ruled out for the rest of the season with a serious knee injury.
The defender ruptured a cruciate ligament in the 3-1 defeat to Barnsley, underwent successful surgery and will look to return to action at the beginning of the 2018/19 campaign.
A trip to Brentford awaited The Lions after the second international break of the season and whilst Romaine Sawyers' strike early into the second half settled matters, a highly controversial moment stopped Millwall from gaining a point at Griffin Park.
Jed Wallace was brought down inside the box and - despite the ball being put into the back of the net instantly - referee Lee Probert decided to give Millwall a penalty-kick. Lee Gregory stepped up and struck well, but his effort was saved by Daniel Bentley.
Neil Harris bemoaned his side's lack of "ruthlessness" after the loss.
"I'm disappointed with some key decisions, but I save most of my frustration for my players, because when you come away from home against a footballing side like Brentford, you want to keep a clean sheet," he said. "We should also be good enough, clinical and ruthless enough to score goals and we wasn't.
"Individually, there were some really strong performances, but it's hard to praise my players when you've just lost 1-0 in a London derby. It's a game we should've got something out of. The players will sit on the bus home disappointed because they threw away at least a point, if not three."
A chance to put things right was presented in front of the Sky TV cameras the following Saturday as Birmingham City came to South London.
Ryan Tunnicliffe, drafted into the side at the expense of the injured Shaun Williams, grabbed his first Lions goal to secure a 2-0 victory after Maxime Colin turned the ball into his own net at the beginning of the second half to open the scoring.
"Birmingham came to compete and I thought we put in an outstanding Millwall performance," claimed Harris. "Despite the odd spell, we were on the front foot for the whole game.
"I said to my players before the game that as much as it will be a fight, a scrap and a battle - and all about second balls - I also knew that there'd be opportunities for moments of quality during the game. I thought we had the key moments, especially in the play leading up to the goals."
Jordan Archer was the star of the show on 28th October as Millwall were once again involved in controversy in a battling goalless draw at Cardiff City.
The stopper was called upon multiple times to keep The Lions' sheet clean, but at the other end, seconds into the second half, Gregory went down under the weight of a Cardiff challenge. A penalty-kick and a red card looked the likely outcome, but referee Mr Martin was unmoved and waved play on - something the manager did not agree with.
"We should've had a penalty at the beginning of the second-half - no question about it. It's a foul and it possibly changes the game if it's given. We could be coming away with three points and they're down to 10 men."
Tom Elliott's fourth goal of the season was not enough to prevent Millwall tasting defeat at Sheffield Wednesday in the final game of the month on Halloween.
Adam Reach gave The Owls an early lead on the night, but Elliott converted on 13 minutes to level the scores.
However, Jordan Rhodes scored the winning goal just before half-time to give Wednesday all three points. The Lions had Jake Cooper sent off late on for a second bookable offence.