The Union go into Thursday night’s MLS playoff conference semifinal at Atlanta United as clear underdogs. Manager Jim Curtin was the first to say it after Sunday’s historic win over the New York Red Bulls, as he sized up the reigning champions’ massive array of attacking firepower.

But Curtin’s players will enter the hothouse atmosphere of Mercedes-Benz Stadium with a notable card in hand: they’re undefeated against Atlanta this season. The first meeting, on March 17 in Atlanta, was a 1-1 tie; the second, on Aug. 31, was a season-defining 3-1 win in Chester.

The playoffs are a different beast, and the lineup for the March game at Atlanta was different from the one that’s expected this time. Three of the players are no longer with the Union — Cory Burke was loaned to a Jamaican club because of visa issues, Derrick Jones was sold to Nashville, and David Accam was sold to Columbus — and reserves Warren Creavalle and Auston Trusty started.

For a game like this, though, the win and tie mean a lot.

“Are there certain little intricacies from that game that give us some ideas of what they like to do, their strengths and weaknesses? Absolutely,” Curtin said Wednesday on a conference call from Atlanta. “But it’s 30-plus games now since that first meeting in Atlanta. The more recent one is within the last month at home, [and] I think our guys can look back on that with confidence and say, 'Yeah, we went in [to] a very big game against them at home, in our stadium, with a lot on the line, and were able to beat them.”

Midfielder Brenden Aaronson should feel especially confident. The Medford native, who celebrated his 19th birthday on Tuesday, scored two of his three goals this season against Atlanta.

“It is an atmosphere that rivals anywhere — and I’m not just saying in the country, I’m saying in the world, it gets that loud,” Curtin said. “You can’t really prepare for it, and for an 18-year-old to step on the field with the confidence that he did on that day and score a goal … Moments like that define professional athletes, and Brenden took his chance and ran with it.”

We’ll have to wait until an hour or so before Thursday’s kickoff (ESPN2 and ESPN Deportes’ TV broadcasts start at 8 p.m.) to find out what Aaronson’s role will be. But we already know that some key players won’t be on the field.

Atlanta will be without starting centerback Miles Robinson, who strained his left hamstring while with the U.S. men’s national team this month; and his replacement, Michael Parkhurst, who suffered a dislocated shoulder in Saturday’s first-round win over the New England Revolution.

The Union, meanwhile, are almost certain to be without leading-scorer Kacper Przybylko because of a foot injury sustained before the regular-season finale.

“He traveled with the group [but] is still unable to participate with the team in training, which is obviously disappointing for him,” Curtin said. "It’s highly doubtful now, at this stage, that Kacper will be able to go. There’s just too much pain in the foot.”

Thursday night’s winner will host the Eastern Conference final on Oct. 30, because No. 5 seed Toronto FC upset No. 1 seed New York City FC on Wednesday at Citi Field, 2-1.