Jake Cooper says Millwall were taken aback by a freak goal on Saturday afternoon as The Lions were held to a 1-1 draw at The Den by Hull City in the Sky Bet Championship's final match of August.
The Lions took an early lead through Jed Wallace's penalty, before Kamil Grosicki levelled just minutes later with a dipping cross-shot 45 yards from goal.
Despite playing a dominant second half, Neil Harris' men were unable to force a way past Grant McCann's Tigers, and the points were shared.
"Certainly, I think we dominated large parts of the game," Cooper says.
"Their goal took us a bit by surprise, and we didn't really react well towards the end of the first half. We picked ourselves up after half-time and performed well, and I think we thoroughly deserved a second goal.
"But our attacking threat wasn't quite there, and we didn't create as many chances as we should have done. We did have the chance with Lenny [Ryan Leonard] early in the second half which would have changed the game had we scored, but I think our first-half performance was frustrating and something we have to look at."
The free-kick netted by Grosicki against Polish compatriot Bartosz Bialkowski was the first goal scored by an opponent at The Den this season, and Cooper confesses the team suffered from the shock of the equaliser.
"The goal shocked us a bit. We've been defensively very solid here at The Den, and I think most of us thought that we could take the game away from Hull after going in front.
"I think the shock of it knocked us as a team - not individually in any way. But it's definitely a lesson we have to learn so that it doesn't happen again.
"I don't know what [Grosicki] tried to do - I certainly wasn't expecting him to try and shoot from there. If he has meant it, then he's pulled off a worldie and fair play to him. If he hasn't then it's just unfortunate for us, and I'm sure Bart wasn't expecting him to try and put it in the top corner from there either."
Cooper has outlined the positives from the first six matches of the season, and claims the squad will remain in good spirits after the first international break.
"We were saying in the dressing room not to get carried away with what we've done so far.
"Yes, maybe we could have picked up more points in our last two League games, but we are who we are and we've only had one defeat from the first six games. It's a credit to us and we've started well.
"Over the international break the lads who stay here will work on things, and we'll look to come back after that and continue to get stronger. The manager talked about working on different aspects over the international break, and we'll certainly come back full of confidence following this first period of the season."