The United States' back line will have enough to deal with between the 105,000 fans, altitude, smog, Javier "Chicharito" Hernandez and Giovani dos Santos. But nothing and nobody will be as much of a terror for the U.S. defense as Andres Guardado.
Guardado is in absolutely scintillating form for Valencia right now and has carried that form over to Mexico. With his breathtaking pace, surprising strength for a 5'7'' winger and ability to cut in as well as get to the endline and put in a cross, Guardado is the best winger in CONCACAF. That he can drop deep to receive the ball, push high or even cut in makes it almost impossible to starve Guardado of service, too.
Unfortunately for the U.S., that is the kind of player, and position, they are most vulnerable to right now.
Injuries to Steve Cherundolo and Timothy Chandler forced the U.S. to turn to Geoff Cameron at right back, an excellent player and maybe the the Americans' best defender, but hardly the ideal choice to face Guardado. Cameron's pace and quickness in space will be tested by Guardado and his physicality will be someone mitigated by the shifty winger. Moreover, while Cameron is playing some right back for Stoke City and not centerback (where many think he would be best), he isn't playing there exclusively. He is also playing left back and defensive midfield, which isn't ideal for a player who never played right back until moving to Stoke a little over a year ago.
Cameron is still learning at right back, and will get a lesson in trying to wrap up Guardado, at the Azteca, in front of 105,000 fans, with the smog, and at altitude. That's hardly what the U.S. wants, and that will put the onus as much on Omar Gonzalez, Michael Bradley and whoever else is in the central midfield to come over and help Cameron out. Guardado is too good to be tied down by one man, and Cameron is going to need the help.