We say: Philadelphia Union 1-2 Nashville SC (a.e.t)
There is very little difference between these sides, as Nashville only conceded two goals fewer than Philly in the regular season, and both teams are well-coached and extremely patient, while rarely allowing you a lot of good looks at the goal.
Nashville will have a slight edge because of how well they have played when trailing, not to mention possessing an x-factor in Hany Mukhtar, who has shown that he can completely change the complexion of any game.
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