Oh, heavens! Who will start along the U.S. back line Friday?

Tempting though it is to draw a big red circle around the severely thinned U.S. defense and say, “Only Jurgen Klinsmann knows what the American back four will piece together in two qualifiers ahead,” there’s a harder truth to consider here:

The U.S. manager probably doesn’t know either.

But he’ll sort it out over the coming days; next week’s match in Mexico is really just for “bonus points;” no one expects to go into Azteca and get a result, although Jamaica just showed it can be done.

BUT! Friday’s contest in Costa Rica is a must-have for all the reasons we have talked about. Long story short, if Klinsmann’s team ties or loses Friday outside Denver, his boys are staring at the very ugly, and all-too-real possibility that the United States national team will have one or no points after three matches in final round World Cup qualifying – a fine Brazilian pickle, if only for the blow in confidence and the fan and media outrage sure to follow.

The U.S. effort to reach a seventh consecutive World Cup would then hang in the balance.

(MORE: Klinsmann discusses roster on national teleconference) 

Back to the battered back line …

Not to put too fine of a panicked point on this thing, but the six defenders named to Klinsmann’s 23-man roster have a total of 12 World Cup qualifier matches of experience. Half of those are from Geoff Cameron, and he was barely on the team at this point one year ago.

Still, the center back situation is not dire. It’s still young, as noted, but talented.

Cameron (pictured) and Omar Gonzalez, Klinsmann’s preferred pairing until further notice, apparently, is available. Matt Besler is untested in qualifiers but held up in the January U.S. camp. Clarence Goodson is a veteran with plenty of experience; he’s not a world-beater, but you know what you’ll get with Goodson, and that’s pretty solid at home against Costa Rica.

But would Klinsmann move Cameron to the outside, where he plays with Stoke City?

If so, would Maurice Edu dropped into a center back spot, where he’s played before. (A little less likely with the late injury news to Danny Williams, which thins the central midfield corps.)

As it stands, Tony Beltran and Justin Morrow are the only natural fullbacks on the U.S. roster. Morrow is a left back, and that looks like the particularly interesting position. There are plenty of options, although they are all highly imperfect.

(MORE: Injury dented U.S. roster announced Monday)

Brek Shea has played as a left back previously, although hardly extensively. DaMarcus Beasley has filled the role at national team level. Both are on the roster just announced.

Or Klinsmann could throw down a wildcard, the way he did with Jose Torres last year against Antiqua and Barbuda. Torres was effective as an attack-minded outside presence in that one. Clearly, however, Costa Rica is not Antiqua and Barbuda; the Ticos will demand far greater defensive attention.

There is one combination (but only one) available from this roster that would have played together as a foursome. Beltran, Gonzalez, Besler and Morrow started in a January friendly against Canada.

Given that such a foursome would mean excluding Cameron, the second-most experienced back line man at this point – And hows about that! – such an arrangement seems unlikely. Translation: a high probability occurrence of a U.S. back line that has never played together. That is hardly ideal.

In fact, we just saw one of those down in Honduras … everyone remember the defensive fiasco where that one fell to pieces?

The subplot here is Carlos Bocanegra’s absence from Monday’s roster. It’s fair to ask if Bocanegra could assist  here? He has, after all, been a starting center back and left back at international level for a decade.

There’s really no getting around this one. There can be but one reason Klinsmann declined to select the veteran of English, Scottish, French and German leagues (not to mention MLS): The manager believes his former captain’s speed and/or skills have declined to the point that he simply cannot be effective at top level anymore.