Ferguson was bewildered by his players' approach against Basle in midweek.
He said: "The way we are playing at the moment there's a cavalier attitude to it.
"Maybe people enjoy it but it certainly doesn't win leagues so we have to do better defensively.
"People say our performance on Tuesday was careless and we underestimated our opponents and maybe there's an element of truth in that. I certainly wasn't happy, even in the first half I could smell we were too relaxed and we could have lost three goals in the first half."
Ferguson pointed out that when Basle scored their second goal, a header by Alexander Frei, United had only one player in the penalty area challenging the striker.
He added: "We were too cavalier. On the second goal for instance we had only one defender in the penalty box. Patrice Evra was out wide tracking the boy who crossed it, Fabio was playing outside right, Phil Jones was in centre midfield. There was only Rio Ferdinand in the penalty box.
"I know it was a fantastic cross but it's an unfair advantage to only have one defender in the penalty box.
"We have to balance attack and defence and that's why it's called a team. A team functions in all parts, understanding each other's parts and we're too cavalier at the moment."