On Sunday, Atlanta United faces off against New York City FC at Yankee Stadium in the first round of the 2021 Major League Soccer playoffs.

Why it matters: Atlanta United won its first (and thus far only) MLS Cup in 2018, but sat out last year's tournament.

• If you don’t follow the team, don’t worry, we’re here to help, with the help of our friends.

Who to watch: On the Atlanta side, it’s Santiago Sosa, a defensive midfielder who’s been out since an injury since Oct. 20th — the last time the Five Stripes played the New York team.

• “Atlanta has felt his absence keenly in recent weeks,” Payson Schwin, a Dirty South Soccer contributor, tells Axios. “Sosa will need to step up big to support his backline as they cope with a potent NYCFC attack that features MLS MVP candidate Valentín ‘Taty’ Castellanos.”

Don't forget Josef Martinez, Jillian Sakovits, an MLS and Atlanta United host and sideline reporter, tells Axios.

• The 2018 MLS MVP is coming off a golazo against FC Cincinnati that secured Atlanta United a playoff spot. "Josef during the playoffs is a whole other beast," Sakovits says. "I am eager to see what [Martinez] has in store after he and his team missed the 2020 playoffs completely."

Know their weak spots: Key NYCFC defensive players are out due to injuries.

• Atlanta must contain Castellanos and midfielder Maxi Moralez, says Jason Longshore, an Atlanta United commentator and host of Soccer Down Here. If they do so, "[the team] will absolutely have opportunities to get at the NYC defense."

New terrain: Yankee Stadium is a unique venue. "Needless to say, soccer wasn't meant to be played there," Sydney Hunte, a Dirty South Soccer contributor, tells Axios. "[B]ut on NYCFC's end it's been able to use that to its advantage."

Know your tunes: Fan groups Terminus Legion and Resurgence have a handy round-up of supporter chants.

Who to follow: In addition to the sources above, check out Felipe Cardenas, Doug Roberson, Rob Usry, among many others.

What’s next: The fun starts at 3pm on ABC and ESPN Deportes.