It won't be before October that Castillo might get a call-up. But he has made a case for getting his shot by playing well for Tigres in Mexico's First Division since moving on loan from Club America this summer and being reunited with Coach Daniel Guzman.
Castillo had a shot at playing for Mexico's Tri in World Cup qualifying -- which would have precluded his ever playing for the USA -- but he lost his passport and never made it to Honduras for the 3-1 loss that cost Sven-Goran Eriksson his job in April.
Bradley has used Heath Pearce, DaMarcus Beasley, Carlos Bocanegra and Bornstein at left back -- none for more than two games.
Whom Bradley uses on Wednesday against Trinidad & Tobago remains to be seen. With Oguchi Onyewu back from suspension, Bocanegra could slide back to left back -- where he plays for French club Rennes and where he has started for the national team since the Confederations Cup.
But with Jay Demerit again unavailable -- Conor Casey was the only other player from the original 22 not on Monday's flight to Port of Spain -- that would mean Chad Marshall would probably keep his spot in the middle of the defense alongside Onyewu against the Soca Warriors if Bocanegra moves to the left side.
Left back has been a problem spot on the U.S. national team for more almost a decade.
Frenchman David Regis held the job through qualifying for the 2002 World Cup but lost his starting job in South Korea, where Frankie Hejduk was the unlikely fill-in for most of the USA's run to the quarterfinals.
Eddie Lewis was the stand-in at left back when no one grabbed the job in 2006, but that experiment was exposed during the disastrous 3-0 loss to the Czech Republic that opened the 2006 World Cup, and Bocanegra took over for the next two games against Italy and Ghana.
What makes the left-back job increasingly tricky in the U.S. scheme -- and why Bradley would be very reluctant to try Castillo there during qualifying -- is that Landon Donovan has moved to the left side of midfield since the emergence of Charlie Davies in Donovan's former position as the second forward.
The switch gives Donovan more opportunities to see the ball and take on defenders. Against El Salvador, he was given plenty of freedom to move forward -- he set up both goals in the 2-1 U.S. win with pinpoint passes -- but it put enormous pressure on Bornstein to man the left side with help from midfielders Michael Bradley or Benny Feilhaber providing cover. But with Michael Bradley in particular often moving deep into the Salvadoran area, Bornstein had lots of space to cover.
Whoever is at left back -- Bornstein, Bocanegra or perhaps Castillo -- will have plenty of responsibility, and raises the question of if and when the New Mexico product, who moved to Mexico as a teenager and represented Mexico at the under-23 level, will get a shot.