IRVINE, Calif. — Every World Cup contender has a pressure point. For some, it’s goalkeeping. For others, it’s a shaky back line or an inability to finish chances. For the United States, it’s the midfield. Not because the Americans lack talent. Quite the opposite. Tyler Adams, Weston McKennie and Malik Tillman form one of the strongest midfield trios the U.S. has ever taken into a World Cup. The problem is what happens if one of them disappears. That’s where the foundation begins to crack.