MLS Cup conference semifinals preview: Everything you need to know

And then there were four. With Round 1 of the MLS Cup playoffs in the rearview mirror, and Toronto and Seattle making it through on Wednesday, Jason Davis and Arch Bell detail the two conference semifinal matches on Thursday night. Who will move on to the final four?

Jump to: LAFC vs. Galaxy

East: 2. Atlanta United vs. 3. Philadelphia Union

Thursday, 8 p.m. ET (watch live on ESPN2)

Backstory: Atlanta took the first step toward a repeat on Saturday with its 1-0 win over New England, but there's something of an ominous vibe around the champs. The loss of Miles Robinson before the playoffs started and the late-game injury to Michael Parkhurst suddenly haveFrank de Boer dealing with a defensive crisis.

Ten seasons in, the Union have their first playoff victory. The incredible comeback nature of that win has only lifted Philadelphia higher as they head to a hostile environment to take on last year's champions. Jim Curtin's team has confidence and a health advantage on their side, and that makes it dangerous.

Playoff path: It was never going to be easy against a Bruce Arena-coached team. The narrow victory against the Revs did belie the chances Atlanta managed to generate, particularly through Ezequiel Barco and Josef Martinez. There should be no doubt that Atlanta is the favorite Thursday.

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The Union had effectively zero momentum going into the playoffs and still managed to fight back from two goals down on two occasions to beat the Red Bulls 4-3. In contrast to the remaining teams in the East, Philly is a side with no true stars -- four different players found the net in the win over New York.

Connecting thread: Few teams in MLS are lucky enough to have one player who represents the club's heart and soul. As different as they are, Atlanta and Philadelphia each have that player in Martinez and Alejandro Bedoya, respectively. Whatever goes down Thursday, those two figure to be at the center of it.

Tactical contrast: De Boer's shift late in the match against New England allowed goal scorer Franco Escobar to range forward and find space behind the defense, indicating a tactical flexibility that will likely come into play Thursday. Atlanta will control possession and try to play to the feet of Barco as much as possible.

Against the Red Bulls, Philly moved to a system in the second half that allowed it to spread out a tired team. On the road in Atlanta, Curtin might take a more conservative approach, sitting in the 4-4-2 diamond to slow down Atlanta's transitions through midfield.

Headaches: Center-back injuries. Already missing Robinson, Atlanta could be without Parkhurst for the remainder of the playoffs after the veteran defender dislocated his shoulder Saturday. De Boer will have to get creative to cover for the pair of absences by leaning on United's depth.

Andre Blake's form is a concern going into Thursday's match after the former Goalkeeper of the Year struggled against the Red Bulls. The comeback win overshadowed Blake's poor day, but it seems unlikely he can get away with a similar performance against Atlanta United.

Star man: Everyone knows about Martinez's goal threat, but the key to Atlanta unlocking Philly's defense and advancing to a conference final is Barco. The Argentine attacker came into his own this season and played the pass that led to Escobar's game winner against the Revolution.

Marco Fabian was supposed to be more than a bench player when he arrived in Philadelphia. His winner against the Red Bulls shows he still has the ability to change the game, even if he's relegated to a substitute role. If Philly is going to spring an upset in Atlanta, Fabian will need to do it again.

Where it will be won: United's wing play can be devastating, thanks to the talents of players like Justin Meram and Julian Gressel. With Barco's abilities in the open field, the game could be decided by the Union's ability to slow down that trio and prevent Martinez from getting free runs that result in scoring chances.

X factor: Darlington Nagbe. Although Nagbe's role doesn't require him to be a goal threat, he is known to occasionally pop up with a surprise shot. The midfielder came close against New England and his ability to test Blake from distance with a low, hard shot could be a factor.

Ilsinho's exploits off the bench are near-legendary in Philadelphia at this point. With his ability to beat defenders in one-on-one situations, the Brazilian can change a game at a moment's notice. If the Union are in the game in the second half, Ilsinho will have something to say about the outcome.

Atlanta will win because: The unrelenting energy of its attacking group. At their best, United come at teams in waves that are difficult to handle for even the best defenses. Philadelphia has momentum, but this Atlanta team has home-field advantage and more than enough weapons to put them to the sword.

Philadelphia will win because: The energy from Sunday's win carries over to a full team effort at Mercedes-Benz Stadium. Blake stands on his head. Ilsinho and Fabian conjure magic off the bench. It's the playoffs, and anything can happen.

Prediction: Atlanta United 2-1 Philadelphia Union -- Jason Davis

West: 1. LAFC vs. 5. LA Galaxy

Thursday, 10:30 p.m. ET (watch live on ESPN)

Backstory: LAFC has been the best team in MLS all season and arrives into the encounter as the top seed in the West after a 72-point regular-season haul to set a league record. It has been a roller-coaster season for a Galaxy side that have their fair share of shortcomings, but still with an unwavering belief that when it comes to facing their city rivals, they will never lose.

Playoff path: LAFC begins its second postseason campaign with a full 18 days of rest, for better or worse, and a chance to extinguish the ghosts of last year's wild-card loss to Real Salt Lake. The Galaxy won for just the second time in their past 10 road games in Sunday's 2-1 win over Minnesota United in Round 1.

Connecting thread: With LAFC still new to MLS waters, this marks the first playoff match between the two; however, there is plenty of familiarity. This rivalry is barely 18 months old, but for LAFC fans, it feels like a lifetime. Five games, no wins for the boys in black and a lot of heartbreak dished out by Galaxy superstar Zlatan Ibrahimovic.

Tactical contrast: As it has done all season long, expect LAFC to be the team to own the ball in this one, with Eduard Atuesta pulling the strings in midfield to set up the likes of Latif Blessing, Carlos Vela and Diego Rossi against a Galaxy defense that has plenty of holes. However, as they showed in the teams' 3-3 draw earlier this season, the Galaxy can put teams to the sword when they break in transition. If LAFC get caught out, a historical season could come to a crashing halt.

Headaches: The injury to midfielder Mark-Anthony Kaye while on international duty with Canada was a setback for LAFC coach Bob Bradley, but he does have league's deepest team. If Kaye is not recovered, there are several options to turn to, with Lee Nguyen the best choice.

After doing some reshuffling for Sunday's affair in Minnesota, Galaxy coach Guillermo Barros Schelotto will likely make several chances for Thursday, and one player who could find himself back in the starting XI is Romain Alessandrini. The Frenchman looked great as a sub against Minnesota in his first match back from injury, and it leaves Barros Schelotto with the hard decision of going with Alessandrini or Uriel Antuna in attack.

Star man: In the history of match previews, there has never been an easier answer. Vela had 34 goals and 15 assists for LAFC in 31 games. Ibrahimovic scored 30 goals and dished out seven assists in 29 games for the Galaxy. Fin.

Where it will be won: It is all going to come down to the Galaxy's ability to defend LAFC's suffocating attack. LAFC completely overwhelmed the Galaxy in their last meeting in the second half, and if not for some last-ditch clearances from the Galaxy and wayward finishing, LAFC would have broken the spell. Also, if LAFC goes up, it will be crucial to keep its composure because the slightest hint of a Galaxy comeback could summon the ghosts of Traficos past.

X factor: For all the talk and attention that Ibrahimovic warrants when these two teams meet, Jonathan dos Santos is the key cog in the Galaxy wheel. The Mexico midfielder was running the show when the Galaxy leaped out to a 3-1 lead in their last meeting, but was then kept under wraps in the second half. The more dos Santos touches the ball for the Galaxy, the better.

Blessing didn't score his first goal against the Galaxy until the 3-3 draw in late August, when he notched a brace, but he has always been a thorn in their side. His movement and ability to get in past defenses wreaks havoc and creates the kind of chaos in front of goal that Vela feeds off in these games.

LAFC will win because: The mistakes that have felled the Galaxy defense will rear their head against the league's best attack and LAFC finally breaks itsTrafico curse.

Galaxy will win because: The Galaxy run their offensive transition to perfection and use their existing mental edge to continue LAFC's Trafico misery.

Prediction: LAFC 5-2 LA Galaxy -- Arch Bell