The Orange and Blue dominated New York in Game 1 by a 3-0 score before rallying from behind to take Game 2 in a tense penalty shootout 8-7 with Jesus Moreno scoring the winner.
For the Union, they followed a similar path, a decisive 3-1 victory in Game 1 was followed up by a 1-0 win in Game 2 with Chris Donovan netting the only goal of the match.
The Orange and Blue will be without MLS Defender of the Year Matt Miazga, who is suspended due to yellow card accumulation. Ex-Philadelphia man Alvas Powell will take his place.
Another former Union player, Ray Gaddis, is expected to be inserted into the starting lineup as well as Yuya Kubo with Santiago Arias and Dominique Badji dropping to the bench.
Left-back Kai Wagner is serving the second of his three-match suspension with Nathan Harriel again stepping in for him.
There is better news on forward Julian Carranza, who used the nearly three-week break to make a full recovery from a hamstring injury and return to the starting lineup.
Defender Jakob Glesnes is also set to return less than a month after undergoing sports hernia surgery but he will only be named to the bench.
FC Cincinnati: Celentano; Murphy, Powell, Mosquera; Barreal, Kubo, Moreno, Gaddis; Acosta; Boupendza, Vazquez
Philadelphia Union: Blake; Harriel, Elliott, Lowe, Mbaizo; McGlynn, Gazdag, Martínez, Bedoya; Carranza, Uhre
Ones to watch
Luciano Acosta (FC Cincinnati)
Almost certainly the MLS MVP, Acosta put together one of the best seasons in league history, scoring 17 goals and picking up 14 assists, the Argentine will again be the focal point of the opposing defense.
Acosta scored from the penalty spot in Cincinnati's win over Philadelphia earlier in the season.
Julian Carranza (Philadelphia Union)
While Quinn Sullivan did an admirable job in Carranza's place, the Argentine's production can't be replaced and his return to partner Mikael Uhre in attack along with Gazdag in midfield gives Philadelphia their formidable attacking trio back in place.
On April 8, Cincinnati defeated Philadelphia 1-0 as Acosta scored from the penalty spot in the 69th minute to extend the club's unbeaten run to seven games to open the season.
The Argentine stepped up to the spot to beat Blake for the only goal of the match, Cincinnati's third consecutive 1-0 win. Roman Celentano made four saves.
September 16 saw the Orange and Blue stage a second-half rally to earn a point in a 2-2 draw and avoid a second straight defeat.
The lead was doubled on 37 minutes, Celentano gave away a penalty after a foul in the box and Gazdag stepped up to the spot to make it 2-0 and remain perfect in penalty kicks since joining MLS.
Four minutes into the second half, Kubo sent a pass towards the Union net and Boupendza was on hand to tap home to cut the deficit in half and he turned provider, feeding Brandon Vazquez, who tapped home following a Philadelphia turnover.
The match will be streamed nationally on Apple TV + with Steve Cangialosi serving as the play-by-play announcer and Danny Higginbotham as analyst.
Kickoff is set for 8pm Eastern time.