The teams earning first-round byes then will play a two-game, home-and-home conference semifinal series that would begin either Oct. 29 or Oct. 30.
The 10 MLS playoff teams have been determined, but the order has not.
Four teams are vying for the top three Eastern Conference positions. The Union and Sporting Kansas City each have 48 points, followed by Columbus with 47 and Houston with 46.
The Union's regular season ended with Thursday's 1-0 loss to the New York Red Bulls, who gained a wild-card berth with the win.
Here are what the other teams are doing this weekend.
Saturday: Kansas City at D.C. United, 7:30 p.m.; Columbus at Chicago, 8:30. Sunday: Los Angeles at Houston.
Here are the scenarios in which the Union would avoid the wild card:
If Columbus loses.
If Houston loses or plays to a draw.
If Kansas City loses by more than one goal.
If Columbus ties and Kansas City either ties or wins.
Tiebreakers could come into play.
The first tiebreaker is head-to-head. If more than two teams are tied, record against each other takes precedence.
For instance, if the Union, Kansas City, and Columbus have 48 points, Columbus wins that tiebreaker because the Crew were 2-2 against both K.C. and the Union. K.C. and the Union were both 1-1-2 in the three-team series.
Yet if the Union and Columbus are tied at 48, the Union win the tiebreaker. The teams were 1-1 against each other, and the Union are ahead in the second tiebreaker, total goal differential, at plus-eight.
If the Union and Kansas City are tied, it will come down to goal differential because the teams tied in their two meetings this season. Kansas City would not only have to lose, but lose by more than one goal for the Union to go ahead. K.C. has a goal differential of nine and the Union's is eight.
The only way the Union could win the Eastern Conference title is if Kansas City loses by more than one goal, Columbus loses, and Houston loses or plays to a draw.
One thing that can't be comforting to the Union is that Kansas City, Columbus, and Houston play teams that have nothing to gain by winning.
When New York qualified, D.C. United and Chicago were eliminated from the playoff picture. Houston is playing a Los Angeles team that already clinched the Supporters' Shield with the top regular-season record.
Another game for the Union to watch is Colorado at Vancouver on Saturday at 7 p.m. Colorado has 46 points and has clinched a wild-card berth. If Houston wins; Columbus wins; Kansas City wins, ties, or loses by one goal; and Colorado wins, the Union would be the No. 3 wild-card team and would open on the road at Colorado, which would clinch the second wild card with the win.
Contact staff writer Marc Narducci at 856-779-3225, firstname.lastname@example.org, or @sjnard on Twitter.