Portugal's heavy defeat at the hands of Germany (currently 3-0 as I type) will provide a massive advantage to the winner of Ghana vs. USA this evening. If there is a winner in that contest, that team will go 3 points clear of their two expected rivals for second with a large advantage in goal difference over Portugal. If the US manage to get a big result in Natal, it would mean that in all likelihood a point against Portugal would be enough to put the them into the knockout stage on 4 points. A draw with Portugal would put the US 3 points clear going into the final day with at least a 4 goal advantage (make that 5 as Muller has just added a 4th for Germany) on goal difference as things stand now. That would leave the US simply needing a close defeat to Germany to get through barring Portugal don't pick up a lopsided win over Ghana.
Knowing that a draw could be enough against Portugal would provide the US a big tactical advantage. They could maintain a compact defensive shape, forcing Portugal to open themselves up and attack, leaving space for the US on the break. Portugal are not a side that's comfortable playing proactively. They prefer to allow their opponents to control the game, then break quickly down the channels through Ronaldo and Nani. It's not a side littered with midfield creativity and they could therefore struggle when being forced to pick apart a crowded defense. Their three central midfielders in their 4-3-3 formation, Raul Meireles, Miguel Veloso and Joao Moutinho, all sit fairly deep in front of the back four and look to provide more defensive cover than attacking endeavor. Moutinho is a skilled passer but none of the three are creative #10 types that will link play with the center forward. While it may be a frightening thought for fans of US soccer to sit deep with an inexperienced back four, it may be a better option than opening themselves up and allowing Ronaldo the space to dribble forward on the break.
This discussion is of course irrelevant if the US fail to beat a very strong Ghana side but if they can somehow manage that feat they'll have a much better shot at progressing than anyone would have imagined when the groups were drawn.