Atlanta United’s debut in the Champions League must’ve felt like a Costa Rican kick in the teeth.

With owner Arthur Blank watching from the stands, the champions of MLS were thumped by Herediano, the champions of Costa Rica, 3-1 on Thursday at Estadio Eladio Rosabal Cordero in the opening leg of the series.

Atlanta United’s defense, something worked on by new manager Frank de Boer for most of the four weeks before the game, was shredded time after time in open play or on set pieces.

Atlanta United’s intensity, something de Boer and players said they  would need in order to match what was expected from Herediano and the more than 7,000 in attendance, wasn’t evident.

“I knew it would be a difficult game,” de Boer said. “A lot of teams didn’t win here when they’ve played CONCACAF Champions League here. Overall, maybe today we didn’t’ show it, but we have a lot of quality players. Today it wasn’t enough. We have to play better as a team.”

It was a flat performance in an important game.

Now, Atlanta United must find a way to over turn the deficit in the return leg will be at 8 p.m. Feb. 28 at Kennesaw State. The winner will advance to the quarterfinals to play the winner of the series between Mexico’s Monterrey and El Salvador’s Alianza FC. Those teams tied 0-0 in the first leg. The quarterfinals will start March 6 or 7. The first game will be hosted by either Monterrey or Alianza.

De Boer stuck with the 11 players that started most of the team’s four preseason games: goalkeeper Brad Guzan, centerbacks Michael Parkhurst, Miles Robinson and Leandro Gonzalez Pirez, wingbacks Julian Gressel and George Bello, midfielders Jeff Larentowicz, Eric Remedi, Ezequiel Barco and Pity Martinez, and striker Josef Martinez.

The surprise was the inclusion of the 17-year-old Bello over the veteran Brek Shea, who has more experience (nine games) in the CONCACAF Champions League than the remaining Atlanta United players.

Herediano needed less than two minutes to scare Atlanta United when a header from about 8 yards by Jose Guillermo Ortiz went inches wide.

The goal rattled Atlanta United, shaking a confidence that the team struggled to get back.

“I thought we were too disjointed as a team,” Larentowicz said. “I don’t think we did things as one. We were split apart. Got rattled in first couple of minutes and didn’t recover.”

Herediano didn’t miss five minutes later. Gonzalez Pirez attempted to head the ball back to Guzan, but didn’t hit it hard enough. Ortiz jumped on the loose ball and chipped Guzan to give Herediano a 1-0 lead in the eighth minute.

“We knew they would come out firing and flying,” Parkhurst said. “We talked about being safe. It’s disappointing to give up the early goal. Gives them more fire and more belief.”

De Boer said he could forgive that goal because mistakes happen. He was more disappointed with how Atlanta United responded.

Instead of using what the team worked on, the team failed to work together on defense or offense.

“Maybe we were too emotional,” de Boer said. “We wanted as soon as possible our goal. Then you see you make the pitch so wide and so open for the opponent.”

Herediano continued to take advantage.

Robinson, who started strongly, cleared another potential goal off the line in the 24th minute.

Herediano increased its lead to 2-0 in the 33rd minute on a goal by Randall Azofeifa. Robinson and Gonzalez Pirez moved to the left to try stop a counterattack. Neither Parkhurst, Remedi nor Larentowicz filled in the middle. The pass made it through and Azofeifa took advantage of his one-on-one with Guzan to slot a shot low and left into the corner. Parkhurst tried to play Herediano into an offside position, but the pass was hit before he moved up the field.

The goal happened because Atlanta United attempted to press in a situation in which it shouldn’t have, according to Gressel. The group was again disjointed.

“We looked like amateurs with six guys running through,” Parkhurst said. “We lost our shape. Can’t don’t that on the road. It’s disappointing.”

Atlanta United scored a few minutes later, but the goal was called back for offside because Gonzalez Pirez’s shot, which was going in, hit Larentowicz, who was offside. Gonzalez Pirez followed up by putting the ball in the net, but the flag had already gone up.

Atlanta United cut Herediano’s lead to 2-1 right in the 41st minute when a shot by Gressel struck a Herediano player. The deflection happened after the goalkeeper committed, leaving an empty net. Pity Martinez was given the assist, his first with Atlanta United.

Atlanta United’s defense of free kicks, horrible most of the night, was at fault for Herediano’s third goal in the 50th minute. Bello flattened a Herediano player about 20 yards from the goal, giving Herediano a free kick in a dangerous spot. The kick crossed through the penalty box, something that happened a few times, and Esteban Granados beat Gressel to the back post to poke the ball into the goal.

Gressel said they were marking zonally, which makes it difficult for him to get to the back post, but that he should have been there to clear the well-hit kick.

Atlanta United missed two more chances to get another valuable away goal. The first came when Josef Martinez’s header went inches wide. The second came in the dying seconds when Hector Villalba’s shot appeared to hit the post.

Atlanta United flew back to Atlanta immediately after the game.

It has less than a week to solve its issues.

“I’m 100 percent sure we can get our revenge,” de Boer said.