That's the easiest way. If Seattle steals a win at Azteca they move on. The number of MLS teams that have beaten a Liga MX team is tiny – FC Dallas beat UNAM Pumas in 2011 and a week later Sounders did it to Monterrey. On top of that, in the CCL era only two MLS teams have advanced against Mexican competition in the knockout rounds: Seattle over UANL Tigres and Montreal Impact getting by Pachuca last year.
So, it's not easy. It's just the easiest way.
Draw with three or more goals scored by each side
That's not easy. Tuesday's result made it clear how good both Seattle Sounders FC and Club America are, but a run-and-gun match at 7200 feet of elevation is unlikely. Still, Club America is prone to giving up goals. Twice this year they've given up three or more goals (Pachuca on the road, Cruz Azul at home). In 2016 they have yet to win a game in which they've allowed a goal.
Draw 2-2 and win in extra time
Away goal tie-breakers disappear in extra time. The two 15-minute halves would essentially be an even game at a mile-and-a-half of elevation after already playing 90. It would almost certainly be ugly.
Draw 2-2, tie in extra time and advance on penalties
Please no. My heart cannot take this. It's like everything in the path above, but it goes to shots from the spot after 30-minutes of grueling extra time with an even result of any tied score.
It's going to be hard. History is not on Seattle's side.
Seattle is 7th MLS team to draw at home vs Mexican team in 1st leg of CONCACAF series.— Paul Carr (@PCarrESPN) February 24, 2016
None of other 6 advanced (5 drew 1-1; 1 drew 2-2).
The match at Estadio Azteca is Wednesday, March 2 at 5 p.m.