Millwall remain just two places and four points away from the relegation zone after a 1-0 defeat at Graham Potter's Swansea.
George Byers' first senior goal settled the contest two minutes before the break which left manager Neil Harris disappointed with the way his team conceded from a set-play.
Potter's men almost surrendered their lead in stoppage time when captain Shaun Hutchinson flicked a corner towards the back post, but Jake Cooper arrived a fraction too late and failed to poke it into the net.
"I would never expect my team to concede from a set-play at Swansea, I have to be frank about it," said Harris.
"I'm disappointed with the players in that respect, it should be us causing the problems at the other end.
"The goal was another freak one where it bounced up off the ground and into the roof of the net.
"You need a bit more fortune in that situation and we didn't get it down the other end right at the death.
"We hung in there and the chance came at the end, I thought Shaun Hutchinson should have got the header on target and then Jake Cooper got his toe on it but could only put it wide.
"First half we paid Swansea a little too much respect. We were very disciplined an organised against a Swansea side who's movement and rotation was very impressive."
Millwall turn their attention to FA Cup action as they travel to AFC Wimbledon in the fifth round.