Union midfielder Jamiro Monteiro will be back for Sunday’s Eastern Conference semifinal playoff game against Nashville at Subaru Park (5:30 p.m., ESPN and ESPN Deportes), but right back Olivier Mbaizo remains out because of Major League Soccer’s health and safety protocols.

We still don’t know exactly why they were sidelined from last weekend’s first-round win over the New York Red Bulls after coming back from their national teams — Monteiro with Cape Verde and Mbaizo with Cameroon — and we might not find out. MLS teams aren’t supposed to specify what happens to players with pandemic-related health issues unless the players themselves do so first.

But we do know Monteiro was back on the field for the Union at Wednesday’s practice. And while Union manager Jim Curtin said in mid-October that all of his players are fully vaccinated, it’s worth noting that one of Mbaizo’s most famous Cameroon teammates reportedly isn’t.

Bayern Munich’s Eric Maxim Choupo-Moting tested positive for COVID-19 on Wednesday, the German club announced. He had already been in isolation after returning from World Cup qualifiers earlier this month, after being in close contact with someone who had tested positive. Multiple outlets in Germany reported that Choupo-Moting is one of five Bayern players whose pay will be docked for missing games due to being unvaccinated.

Mbaizo was also on that squad, and helped deliver a pair of wins that sent the team to the final round of Africa’s qualifying campaign. Another Indomitable Lion in MLS, Seattle’s Nouhou Tolo — who’s also fully vaccinated — missed his team’s shocking upset loss to Real Salt Lake on Tuesday because of the same rules.

» READ MORE: Jamiro Monteiro and Olivier Mbaizo played in some of the biggest games of their lives in World Cup qualifying

As Curtin answered a question about Mbaizo’s status during a Friday lunchtime news conference, he said, “We’ll do everything we can to get him eligible for the game.” That means Mbaizo hasn’t been officially ruled out yet, though the odds seem slim that he’ll be cleared.

If Mbaizo is out, expect Alvas Powell to start at right back again. The veteran Jamaican was superb in the Union’s 1-0 win over the Red Bulls, his 13th-career playoff contest. He’s also one of only two players in the Union locker room with a MLS Cup winner’s ring, from 2015 with the Portland Timbers. (Aurélien Collin is the other, from 2013 with Sporting Kansas City.)

“It’s incredible to have a player like Alvas on the bench,” Curtin said, also citing his years of experience with Jamaica’s national team. “He knows what pressure is. … To know that there’s another gear and intensity that it has to go to, Alvas stepped up and was excellent for us.”

This will be the third meeting of the year between the Union and Nashville. Each side hosted one in the regular season, and both games were 1-0 wins for the home team: in Nashville on July 3 and at Subaru Park on Oct. 23.

» READ MORE: Union to host Nashville SC in second round of MLS playoffs

It feels like the teams have been looking over their shoulders at each other for weeks now. They battled for the East’s No. 2 seed from the late summer into the autumn, knowing the prize was the right to host this game.

All eyes will be on Nashville’s star playmaker Hany Mukhtar, a leading candidate for this year’s MVP award. He scored two outstanding goals against Orlando after piling up 16 goals and 10 assists in the regular season.

Defending him will be a collective effort, but it will naturally be a particular emphasis for José Andrés Martínez. The Union’s stalwart defensive midfielder drew a yellow card for dissent late in the second half of the Red Bulls game, which means if he gets booked in this game he’d miss the Eastern Conference final.

Curtin wasn’t happy that Martínez ran his mouth, but he said he’ll accept a yellow in this game if it comes for a good reason.

“I don’t mind if José makes a tactical foul or a hard challenge that maybe is on the edge,” he said “We don’t want to obviously lose him, but at the same time, you have to do everything to survive and advance.”­

» READ MORE: Jakob Glesnes’ epic game-winning goal set off one of the Union’s all-time celebrations

Get there early

The Union are encouraging fans to be in their seats by 5:20 p.m. to take part in a pregame light show that will be displayed through fans’ mobile phones. The phone app is available through the Union’s website. All fans in attendance will also get free rally towels.

For the Red Bulls game, fans got free T-shirts and there was a stadium-wide pregame card display as the teams took the field. But because many fans arrived close to kickoff, there was a backup to enter the stadium’s parking lots. So there were a lot of empty seats when the display happened.

» READ MORE: Andre Blake reflects on a tough year as he leads the Union into the second round of the playoffs