Looking at the bracket, it's very important the US wins against Brazil in the final group D match. Right now, we're in first place, and it will be a miracle if we don't qualify (because of the goals we have, even a 3rd place finish almost certainly gets us through), but the second round will likely be a cakewalk if we come in 1st and will likely be the end for us if we come in 2nd.
Here's the bracket:
http://fifa.com/u20worldcup/matches/kostage.html
So, as you can see, the group D winner takes on the 3rd place from B,E, or F, and none of the likely 3rd place teams there seem to be a serious challenge. Here are the groups and standings:
http://fifa.com/u20worldcup/standings/index.html
Of course, the QF and SF will be a challenge (quarters are probably against Mexico or Portugal and semis probably Spain), but at least we avoid Argentina for as long as possible.
On the other hand, if we come in 2nd in group D, we immediately face Argentina in the second round!! So, this last match against Brazil is a critical one and one we really should win.
Actually, coming in 2nd is the worst of the 3 possibilities. Coming in 3rd would be much better than 2nd (we'd then face Chile), but we don't control that.