The U’s saw their unbeaten home record thrown out of the window, as a first half opener from Zan Vipotnik and then a clinical strike from Florian Bianchini put the Swans firmly in control with 10 minutes to go.
Dane Scarlett pulled one back for United with two minutes left on the clock, and despite a frantic and tense finale, a below par U’s were unable to grab a leveller.
The defeat ensured that United didn’t just lose their unbeaten home start, but also extended their winless run to eight matches.
U’s head coach Buckingham said: “I didn’t think we were very good at all today for very large periods of the game.
“Despite not being very good with the ball, we’ve still created three very good goalscoring moments before Dane scored his at the end.
“That’s the disappointing thing. We are so much better than what we’ve just shown there.
“We need to start the game a hell of a lot better than we did, and make sure we believe that we’re good enough to do what we do at this level.
“That’s what I want to see, and that’s what we will see on Tuesday.”
He added: “We’ve had a really good six to eight months. This club has been on the up, and this is a challenge.
“We knew this year was going to be a challenge. It’s making sure that when we hit these challenges, that we’re clear on what we do and how we do it.
“We don’t get carried away and we don’t lose ourselves in these moments, and that’s what I’ve just said in there.
“We will lose games of football, but we’re not losing them that way, no way, not while I’m here.
“We have to make sure that how we turn up Monday to get ready for Tuesday, we’re in a much better place than we were today.
“We’re a closed group, a together group, and we’re clear on what we’re doing. We’ll make sure that we address what we need to address, and we’ll be back Tuesday to put on a good performance.
“It’s been a really good six to eight months, where it’s not gone our way, but we’ve rightly deserved what we’ve got.
“In this league, you get what you deserve. If you don’t do the basic stuff right, you’re not going to put yourself in the best space.”