As he walked toward the locker room last Wednesday, a wry smile broke across Real Madrid manager Jose Mourinho's face. Real Madrid had just drawn 1-1 at home with Manchester United in the Champions League's Round of 16—and you wondered if Mourinho knew something the rest of us didn't.

Conventional wisdom would suggest that United now has the upper hand. United plays the two-legged affair's next 90 minutes—and possibly 30 minutes of extra time and penalty kicks—in front of its own fans. And in soccer, as in most team sports,...