The Philadelphia Union have encountered a roadblock ahead of competing in their first-ever Eastern Conference Final.

Union head coach Jim Curtin revealed Friday that the Union have several players in health and safety protocols due to COVID-19, leading to Thursday's training being canceled before their Audi 2021 MLS Cup Playoffs match against New York City FC on Sunday at Subaru Park (3 pm ET | ABC, ESPN Deportes).

“We were able to return to practice today,” Curtin told media. “We hope to have as many players available as possible for our game against NYCFC.”

Curtin declined to give a firm number on players currently in flux, noting the situation is fluid and can change at any moment.

The Union’s Round One playoff match against the New York Red Bulls came without starters Jamiro Monteiro and Olivier Mbaizo, as both missed due to health and safety protocols. That match was decided by a 123rd-minute golazo from Jakob Glesnes. It took penalties for the Union to advance past Nashville SC in the Conference Semifinals.

Now, the East's No. 2 seed may be down several players for Sunday.

“Our players have worked tirelessly to get to this point,” Curtin said. “I know they’ll be brave even in the adversity that we’re facing. We’ve stepped up in big moments all year long. Be it international absences, the challenges of Concacaf, players losing loved ones. Adversity on and off the field, this group always finds a way to step up. We will make our fans proud.”

NYCFC won’t be at full-strength, either.

The Cityzens lost key players Anton Tinnerholm (ruptured Achilles tendon) and Keaton Parks (leg blood clot) to injury before the playoffs started. Further, Golden Boot presented by Audi winner Taty Castellanos will be suspended after being red-carded in the club’s win over New England on Tuesday.

“Castellanos is a great talent,” Curtin said. “He’s a forward you’d love to have on your team but you hate to play against – I say that as the ultimate compliment. He’s a tireless worker, big part of their success. It’s unfortunate he gets the red card at the end. But they have a lot of good options on their bench. They have strong attackers and an incredible coach, he’s done a great job with this team.”

NYCFC received some good news Friday, though. James Sands, who subbed out with an injury against the Revs, and Tayvon Gray, who barely made it to the final whistle with a clear leg injury, will both be available.

Irrespective of who's on the field for either team, Sunday’s Conference Final is set to be a thrilling game.

“It’s certainly difficult to prepare, it’s unique,” Curtin said. “We usually don’t have an off day amid preparations for the biggest game in our club’s history. I’m not going to lie to you and sugarcoat it and say it’s been easy, but we have jobs to do. We’re all professionals. It’s my job to get our guys ready, there will be no excuses. We’ll go out on the field and give an effort that makes the badge proud.”

Philly, 2020 Supporters' Shield winners and a 2021 Concacaf Champions League semifinalist, aim to make history one more time this year.

"If history has shown us anything, it’s that a group of men, or a group of women, with absolutely nothing to lose, can oftentimes be the most dangerous,” Curtin said.