AIDEN O'Brien couldn't put his finger on why Millwall were so flat on Saturday – and said their defeat to Barnsley was a "wake-up call".
The Lions had won their last three league games at The Den, but the Tykes' 3-1 victory was their fifth successive win against the side they also defeated in the League One play-off final in 2016.
O'Brien admitted Millwall weren't at the races at the weekend.
"We weren't ourselves. We didn't impose ourselves on the game the way we usually do," O'Brien said. "We were reactive instead of proactive, in the knockdowns and stuff we weren't first to the ball.
"We didn't play our game, basically. It cost us, we lost 3-1 at home. We very rarely lose like that at home and we've just got to learn from it now.
"We've got to come back after the international break and show everyone we're much better than that.
"We can't afford to be 50, 60 or even 80 per cent. We need to be 100 per cent if we want to go out and compete in this league. It's a hard league and we can't afford to be slacking in any part of the pitch, we've got to be on it in every game.
"When we are on it we're frightening, as we've shown. So, yeah, it's definitely a wake-up call.
"Everything we did on the training ground we did sharply. We were ready for the game but it just happened that on the day that it didn't happen for us."
Millwall boss Neil Harris confirmed after the defeat that Shaun Williams would be out for three months with a knee injury.
Ryan Tunnicliffe started on Saturday and will be Williams' replacement for that period.
O'Brien says Tunnicliffe can offer something different to Williams, but he admitted his fellow Irishman will be a big loss.
"For me, he's always one of our stand-out performers," O'Brien said. "I always feel with Shaun Williams in the team he gets it ticking, he gets on the ball and is one of our star men.
"Most of the time without him in the team we don't usually get on the ball and make things tick.
"He's definitely going to be a huge miss.
"He can offer a lot, Tunnicliffe, and add a few goals as well. It was his first start in the league so you've got to give him some time to gel with the team. I thought he did well."
Most of the Millwall squad have a few days off early this week. O'Brien, though, was flying to Dublin to join up with the senior Republic of Ireland squad for the first time for their final two crunch fixtures of World Cup qualifying.
Martin O'Neill's side face Moldova on Friday and then Wales next Monday in what could be a shootout between the two countries to secure a play-off place.
"I was buzzing, over the moon," O'Brien said about getting the call-up. "I can't wait to get out there. For me and my family it's massive. I'm going to embrace it and play with a smile on my face, do what I do best."
Image: Millwall FC