And fans turned on the players on Saturday, with particular abuse aimed at Roberto Martinez, Andy Gurney and boss Kenny Jackett.
Trundle told the Swansea Evening Post: "When the crowd turn against some players, it knocks them out of their stride.
"Especially if their confidence is low in the first place. I was a little disappointed with the way some of the fans reacted.
"To sing 'Jackett sort it out' can't be right when you look at the job he's done.
''It's simple really...if they're behind us it helps, and if they're not it doesn't."
The Swans travel to Darlington on Tuesday night, with the game set to go ahead despite the recent cold snap, knowing that a win could reignite their promotion hopes.
Then on Saturday they host United in a top-of-the-table six-pointer.