The Spanish team does feature multiple NBA players at the guard position. Jose Calderon, Rudy Fernandez and Juan Carlos Navarro have all played in the NBA during their basketball careers and are skilled at their respective positions.
Against most world teams, this group's shooting and driving ability would overwhelm the opponent's defense. Calderon is a fantastic floor general and excels at setting up his teammates for easy shots.
Fernandez is a high-level perimeter shooter who spaces the floor for the Spanish team. He also has the ability to attack the basket and either finish or kick to an open teammate. The same goes for Navarro, who is very quick and is effective on fast-break opportunities.
With that being said, it is clear that the Spaniards severely lack the talent and depth of America's guards. Team USA features Kobe Bryant, Chris Paul, Deron Williams, Russell Westbrook, James Harden and other NBA stars.
Each of those players is better than any guard on Spain's roster and will be a matchup nightmare in the open court.
Therefore, it is unreasonable to expect Spain's guards to dominate the United States at that position. However, the guards do not have to carry the team. That is what the frontcourt is for.
In order for Spain to have a fighting chance against Team USA, it needs its guards to simply be effective and not lose the game for their native country. If Calderon, Fernandez, Navarro and others can emerge as a threat from the perimeter, spread the floor with great ball movement and limit turnovers, then they will have done their job and negated some of the athletic advantages of the American guards.