GARY Rowett revealed he had planned to play five at the back at Birmingham before the squad was hit with injuries on Friday.
Shaun Hutchinson missed out with a rib problem as the Lions drew 1-1 with Blues.
Shaun Williams gave Millwall the lead in the 61st minute with a superb 25-yard shot into the top corner but Birmingham rescued a point through Jake Clarke-Salter's close-range finish from Danny Crowley's 79th-minute corner.
Matt Smith had seen a first-half header superbly saved by Connal Trueman.
“I wouldn't say I'm pleased to a point,” Rowett said. “First-half performance was very, very good. I thought we were by far the better side first half and created some very, very good chances. We had a lot of control of the game and when we counter-attacked I thought we did it excellently.
“Birmingham play an expansive style, they're trying to make a lot of passes. But I felt our shape was very good and we were aggressive in terms of pressing the first ball. I didn't think they were able to play through us.
“We had a wonderful chance with Matt Smith. Jed [Wallace] plays a brilliant cross, whips it in with a little bit of pace and normally for Matt that's like a tap-in.
“But it's a great save from Connal Trueman. We went from strength to strength first half and probably should have been a couple of goals ahead.”
Williams gave Millwall the lead with his first goal of the season.
Rowett added: “He pulled out of training with a hip injury yesterday. We had a few walking wounded. We were going to play five of the back like we did at Swansea but we only had four defenders.
“It made my decision pretty easy. But we got through it. It's another tough away game. Second half, did we run out of steam or out of a little bit of composure to get up the pitch, because we kept giving the ball away. And that means then there's a real swing to the game.
“Birmingham, just with bringing [Jefferson] Montero on and allowing him to play wide left allowed them to create a bit of momentum. We're disappointed to concede the goal in the way we did. Our team, our squad with the players that we've got, we shouldn't concede a goal from six yards out from a free header.
“But it’s a positive week with five points from three games, two tough away games. We're respectful and realistic about the type of team we are, but also I think we're just starting to show a real ambition if we can piece it together, some of those 20-minute, 30-minute spells into a whole performance when we're a really good side.
“I asked someone after the game how far are we are off the play-offs and they said five points. The person then started telling me how far we are from the bottom three and I said, ‘I’m not really interested in that’.
“We're looking up because we're good side and we've got good players. We're starting to see some of those players perform really, really well so often.”
Blues boss Pep Clotet felt his side should have had two penalties. Jude Bellingham, who is just 16, went down in the first half and claimed Murray Wallace had made contact.
In the second half Clarke-Salter felt he had been pulled back by Jake Cooper.
Clotet said: “The Millwall player put his hands up and normally that means there was contact.
“The second one was clear. It was not easy for the referee, it was a tight game. Millwall put a lot of players behind the ball, it was not easy to see those details.”
Rowett disagreed with Clotet. “Two penalties? Blimey. I didn’t even think they should have had one.”
Image: Millwall FC