It might be the biggest compliment you can give Atlanta United, the fact that it has so suddenly, so obviously, become so difficult to imagine what Major League Soccer was like before their arrival.

Okay, perhaps “difficult” is an exaggeration. After all, it was only 15 months ago that the Five Stripes were a hypothetical, still awaiting their MLS debut. Fans who can’t remember the league’s landscape before the 2017 season probably haven’t been fans for that long, and they’re probably not digging deep on preview pieces at Timbers.com.

Still, when you see Atlanta’s place at the top of the Eastern Conference, it feels natural. The crowds at Mercedes-Benz Stadium, the ones responsible for the league’s eight largest attendances this season? They make it easy to forget that the venue wasn’t even open when the team debuted last year. Just over one season into United’s on-field existence, there are already elements of the team that feel eternal.

That place atop the East is one of them. No disrespect to teams like the New York Red Bulls, New York City FC, or the actual reigning champions of the conference (and league), Toronto FC, but when you see a team with arguably the league’s best player (attacking midfielder Miguel Almirón), its best scorer (striker Josef Martínez), and its most talented newcomer (attacker Ezequiel Barco), it feels like that team should be on top. Their five-point lead on the East. An attack that’s scored three more times than anybody else, this season. Is anybody truly surprised that Atlanta United is off to such a strong start?

If anything, the losses they’ve incurred have been so rare, so uncharacteristic, they stand out as aberrational. The season-opening 4-0 loss at the Houston Dynamo? That’s what aberrational is all about. The 2-0, May 9 loss to visiting Sporting Kansas City? There was a 34th-minute red card. And Red Bulls’ 3-1, May 20 win at Mercedes-Benz? Atlanta was on the cusp of a 2-0, early lead, before the intervention of video review.

New York earned their result that day, but the circumstances of the game feeds into a more general impression of United. When they’re playing well, that impression says, Atlanta’s not going to be beat. Even when they’re not, opponents need something strange to happen in order to throw the Five Stripes off course.

That’s the context for Portland’s first MLS visit to Atlanta, one which, for diehard Timbers fans, should feel vaguely familiar. After all, the same idea of being an underdog against a nearly invulnerable Eastern Conference contender surrounded New York City’s April 22 visit to Providence Park. At the time, NYCFC sat atop the East. They were undefeated, before that day’s kickoff. But when the final whistle blew, the Timbers had a 3-0 victory, the second game in what’s become a 10-match unbeaten run, across all competitions.

So how do the Timbers replicate that feat? In some ways, that’s not even the most interesting question about Sunday’s match, but let’s indulge it for now, doing so by looking back on how the team conquered New York. As opposed to staying with the system that, the week before, broke through for the team’s first win under head coach Giovanni Savarese, the Timbers tailored their approach for NYCFC, playing a 4-1-4-1 formation for the only time this season. The team also defended very deep, ceding 74 percent of the match’s possession to their guests as they tried to use New York’s tendencies against them. The resulting three-goal victory revealed a team capable of not only adapting to their opponents but succeeding while doing so.

If there’s anything to learn from that example, it’s the Savarese won’t feel wedded to what’s worked. Mentality, he’s stressed all season, has to be his team’s most important trait, within which, whichever 11 players are chosen to start Sunday’s game must be able to execute a potentially brand new approach. In the face of the Timbers’ biggest challenge of the season, Savarese may need to come up with his most unique solution.

“We’re performing, right now, in the best possible way with the players that we have,” Savarese said, in his weekly press conference, before he and his team departed for Atlanta. “For me, the most important thing that we need to accomplish, we have accomplished, and that’s to create a group that is mentally strong. That was the priority.

“Right now, I can see guys who, whoever steps on the field, they do the job. They battle one another in practice to get on the field, so I’m very happy we’ve achieved that part, which was one of our priorities at the beginning of the season.”

Make no mistake about it, this will be Portland’s biggest challenge, yet. Within Major League Soccer, this would be any team’s biggest challenge. In only 15 months, Atlanta’s cultivated one of the league’s most talented teams, performing in one of its most daunting environments, producing MLS’ best results. This is a challenge which, within the context of Portland’s regular season, may prove unparalleled.

But riding a 10-match unbeaten run, what better time than now? With a team that’s as healthy as it’s been all season, as confirmed in Savarese’s press conference on Tuesday, why not embrace this challenge. With over two months passed since Portland surprised New York, it’s time to test how far the team has come.

“I think it comes at a perfect time,” Savarese said, about Sunday’s matchup, “because these are the type of games that you want to play. We have a lot of guys healthy, and we can bring a good team to Atlanta to face them. We’ll measure ourselves and see where we’re at, and hopefully the game goes our way. Because games have momentum, and you never know how they’re going to go.”

It’s that test that leads to what’s truly the most interesting question surrounding Sunday’s game: What if Portland wins? If Atlanta picks up another victory, the MLS world goes about its business, with same competitive hierarchy the league’s established over the season’s first four months. But what if the Timbers win? And they run their winning streak to 11? And we can look back on their results against NYCFC, Los Angeles FC, Sporting KC and Atlanta in the light of their new, most impressive accomplishment?

That changes the context of everything. That would mean Atlanta, whose losses have seemed so aberrational, may be more vulnerable than we thought. But it would also mean this journey the Timbers are on, one that’s moved forward methodically since the second week of the season, may have to be cast in terms of something new.

If Portland can take three points on Sunday in Atlanta, the proving part of their journey may be done. MLS will get a clear picture of what the Timbers have become.