United took the lead at the start of the second half through Juan Mata, but saw their previously impregnable defence crumble under the pressure of Andre Ayew and Bafetimbi Gomis, whose goals sent the Red Devils to their first defeat of the season.
A tactical change from Swansea manager Garry Monk may have swung the game, with Ayew pushed up from midfield into his familiar attacking position. From there, the hosts scored twice within five minutes to claim all three points.
Van Gaal said his side did not react to the tactical change due to their ‘dominance’, and rued his side’s lack of luck in front of goal.
“We controlled the game for 85 minutes but we lost in five minutes and that cannot happen,” he told BBC Sport.
“After 1-0 they changed their shape and then we couldn’t cope with that, so that’s not good.
“They changed the shape and we had to be compact. We forgot that maybe because we are too dominant. We have to learn from that.
“First you have to create chances, with scoring goals you need luck, that we don’t have at the moment.
“I have enjoyed our football but you need the result and we lost again. All the players were very motivated to beat them.”
“You see the game today, we could have scored much more,” he added, to Sky Sports. “I don’t think that’s the problem.
“When the opponent scores goals you can always find errors. I have to analyse that with my players.
“But we have to score more and you can’t concede goals.”
Van Gaal refused to blame Sergio Romero for either goal, with questions asked of the Argentinean after he failed to save Gomis’ winner.
“You can write everything you want,” he said when asked about Romero’s performance. “What I always say is defending starts in front and not only in the last line.
“You can always analyse errors but I don’t think that is a good way.
“You have to see the way the team is defending as a team and then I know what errors has been made.
“At the end, of course, the pressure for the last defender or goalkeeper is always higher.
“I don’t agree when you only say the goalkeeper was not so good.”
Van Gaal insisted he did not did know what the future holds for Real Madrid target David De Gea before the Spanish transfer window closes at midnight on Monday – 18 hours before the English window shuts on Tuesday evening.
“We have to wait and see but I don’t want to interfere,” he said.
“As a manager we have divided our tasks and the task is for Ed Woodward (United chief executive) and not for me.”
When asked by Sky Sports over the potential of United strengthening over the last two days of the transfer window, Van Gaal responded: “When you are scouting players, you know already which players you can buy, it is not the last moment. When you see the game you will see we are the dominant team.
“You are asking me the same questions.”