After Hans Backe had publicly fretted over Chivas USA being a “very, very tricky game,’’ his Red Bull got upset 3-2 at home, mauled on set pieces. Now he’s acknowledged it was probably a mistake to even admit his worries openly, but says his team doesn’t so much need an attitude adjustment as a tactical refresher course.

With his Red Bulls playing May 7 in Los Angeles and last Wednesday in Montreal, Backe expressed concern over not having a single truly intense practice session all week. He turned out to be right, as they came out flat Saturday and allowed a pair of sloppy set piece goals. But he admits voicing that concern may have just given his players a mental alibi to worry themselves.

“It’s mental. As soon as the players see you have a week without sharp sessions, solid tactical work, that sometimes (happens),’’ Backe told the Post. “But I assume it was probably the wrong way that we let it come out talking about this, because it affects everyone.

“Suddenly everyone starts to believe we have a problem, it will be a tricky week. We probably should have tried to not even talk about it, because it’s so easy for players in the week to say “Well, no sessions, of course it can affect you.’ I definitely think we should’ve handled that a little different.’’

Defender Tim Ream agreed that the week without hard tactical work dulled their mental sharpness. Considering Houston is looming this Saturday, that’s a problem they’d better fix posthaste.

“Mentally it was pretty clear from even before the game downs were kind of down, heads weren’t in the game, giving up as goal six minutes in,’’ said Ream. “That’s something we hadn’t done all year. You could tell from the outset mentally we definitely we weren’t into the game.’’

And nowhere was their mental malaise more evident than on set pieces, where they earned an astounding 14 corners but got just one single shot on goal from those, and then saw two Chivas USA free kicks lead to goals after having defended set pieces well all year. After the Red Bulls had tied the score at 1-1, they saw Ante Jazic settle Heath Pierce’s looping free kick and Justin Braun hammer it past keeper Bouna Coundoul for a 2-1 lead.

Then after mustering an equalizer in the form of Dwayne De Rosario’s penalty, the Red Bulls allowed the eventual winner in the 56th minute, when Alejandro Moreno headed Simon Elliott’s free kick off the post and the ball trickled back in front of the goal where Braun tapped it in to complete his hat-trick.

“We use the term “switched-on,” and when you don’t get a chance to train, work on defensive set plays, offensive set plays, when we don’t get a chance to do that we kind of lose focus, kind of forget what you’re supposed to do at the most basic level. It hurt us,’’ Ream admitted after the Red Bulls’ zonal defense fell apart.

“The set-plays were terrible. At the wrong time against set plays, it ended up two or three players marking well and the other two or three just standing still and letting them run in behind. It was poor. I said what is going on,’’ said Backe. But he was confident a crash-course of tactical work this week would fix the problem.

“It’s easy this week to just look at the video, look at the game, and just sharpen _ it should be very easy _ our tactical work, our defending, our defending set plays, our attacking set plays. That’s the only thing to do now, every session, work with this and do it sharp.’’