Neil Harris criticised his Millwall side for switching off after Blackburn grabbed two late goals in a 2-0 victory at The Den.
The game look destined for a draw before substitute Adam Armstrong's cross was deflected by Millwall skipper Shaun Hutchinson, under pressure from Joe Nuttall, into his own net after 86 minutes.
Armstrong then wrapped up Blackburn's first away victory since the start of October with a stunning solo strike, with Ryan Leonard's shot on the hour the closest Millwall came to a goal as they saw a four-match winning run brought to an end.
Defeat leaves the Lions looking over their shoulder, with just five points separating them from the Championship relegation zone, and Harris admitted his side were punished for losing their concentration.
"I'm frustrated, definitely," he said. "That was a 0-0 draw all the way through. I can't really remember many chances at either end.
"I felt that we put some good balls into their box but didn't really attack them, and they had two or three free-kicks and applied a little bit of pressure in the second half. But I never felt like we were going to concede a goal.
"I think that once again it's the reality of where we're at, it's another learning curve. If you switch off at any moment in the Championship, against any side above you in the league, you'll get punished.
"We switched off at 86 minutes and the ball goes in our net. The second goal is disappointing – it's a killer. If you don't concede straight away, you may get that little bit of momentum and that one chance in stoppage time."
Despite his team coming away with all three points, a result which moves Blackburn up to 14th in the table, manager Tony Mowbray was less than impressed with the quality of the game.
"I'm pleased with the points, we'll take those," he said. "It's really hard to come away from home in the Championship and win because there are so many huge clubs in the division.
"Any three points are welcome but I'm not pleased overall, because in my opinion that was a terrible game and I'm glad that I didn't have to pay to watch that this evening.
"We had to prepare to come to The Den and we knew that we'd have to match the intensity of what you get here.
"There's so much data out there which shows that Millwall start games really well, so we had to make sure that our team had their adrenaline levels right and that they were able to match the intensity of what was coming.
"Sometimes when you do that, and make your team ultra-competitive, you forget how to play yourselves and I think that's probably what happened.
"We forgot to pass the ball because we knew we'd have to head the ball and compete in midfield. It wasn't a great spectacle but we'll take the clean sheet, with three centre-halves out, and the points.
"Normally Adam Armstrong would start for us but it wasn't a bad game to name him on the bench and see if he could make the difference late on."