Atlanta United welcome fellow MLS side Philadelphia Union to Mercedes Benz Stadium for their CONCACAF Champions League quarter-final fixture on Tuesday.This will be the first of the two-legged fixtures between the sides, who have last squared off in 2019.The hosts have kicked off the MLS 2021 season on a positive note. They played out a goalless draw in their season opener but recorded a superb 3-1 win over Chicago Fire in their previous outing.The Union were impressive in their last 16 win over Deportivo Saprissa but have scored just once in two league games. They also played out a goalless draw in their first game of the 2021 season but fell 2-1 to Inter Miami at home on Saturday.The quarterfinals are set 👀 What will be the most exciting matchup this round?#SCCL21 pic.twitter.com/taqL0MwIC6— Scotiabank Concacaf Champions League (@TheChampions) April 22, 2021Also See: Top 10 football players with the most fans in the worldAtlanta United vs Philadelphia Union Head-to-HeadThe two clubs have squared off seven times so far. Six meetings have come in the MLS regular season while one was in the playoffs. They have never crossed paths in the CONCACAF Champions League.This is the debut season in the continental competition for the Doops. Meanwhile, The Five Stripes have made it to this stage of the tournament for three consecutive seasons.The hosts have a superior record against their Eastern Conference foes and have four wins to their name. Philadelphia have just one win while two games have ended in draws.They last met in the MLS Playoffs in 2019. Atlanta United cruised to a 2-0 win in that fixture.Atlanta United form guide across all competitions: W-D-W-WPhiladelphia Union form guide across all competitions: L-D-W-WAtlanta United vs Philadelphia Union Team NewsAtlanta UnitedGoalkeeper Alec Kann and midfielder Mo Adams are the two injury concerns for the hosts in this first-leg tie. Kann has a shoulder injury while Adams is expected to be out for another month on account of a hernia injury.Meanwhile, first-choice goalkeeper Brad Guzan will be able to return between the sticks in this game. He picked up a red card in the first leg of the round of 16 fixtures and was suspended for their last Champions League game.Matheus Rossetto has obtained a US green card and can mark his return to the squad, as he will no longer occupy the club's international spot.Still thinking about this banger 🔥 #SCTop10 pic.twitter.com/RCpeCo6hUc— Atlanta United FC (@ATLUTD) April 25, 2021Injured: Mo Adams, Alec KannSuspended: NoneUnavailable: NonePhiladelphia UnionThe only injury concern for Union at the moment is veteran midfielder Ilsinho, who injured himself in the loss against Inter Miami just 11 minutes after coming off the bench.Jack de Vries is recovering from a concussion and might not make the trip.🗣 TUESDAYNext up ➡️ Headed to ATL for Leg 1 of @TheChampions Quarterfinals!#DOOP pic.twitter.com/A6QIiLINBI— PhilaUnion🛡 (@PhilaUnion) April 25, 2021Injured: Jack de Vries, IlsinhoDoubtful: IlsinhoSuspended: NoneAtlanta United vs Philadelphia Union Predicted XIAtlanta United Predicted XI (4-3-3): Brad Guzan; Alan Franco, Anton Walkes, Miles Robinson, George Bello; Santiago Sosa, Emerson Hyndman, Ezequiel Barco; Marcelino Moreno, Jake Mulraney; Lisandro LópezPhiladelphia Union Predicted XI (4-3-1-2): Andre Blake; Jack Elliot, Jakob Glesnes, Oliver Mbaizo, Kai Wagner; José Martínez, Leon Flach, Alejandro Bedoya; Jamiro Monteiro; Kacper Przybylko, Sergio SantosAtlanta United vs Philadelphia Union PredictionPhiladelphia Union have seen a sudden drop in form in the MLS after scoring five goals in the CONCACAF Champions League round of 16 fixtures.Atlanta are unbeaten in all competitive fixtures this season. Given their form at the moment, we do not expect them to drop points at home. A win for the home team looks like the most likely outcome.Prediction: Atlanta United 1-0 Philadelphia