The just-concluded USMNT camp will largely go down as a positive for D.C. United. Perry Kitchen and Steve Birnbaum both got their first-ever senior caps, and given that both are young players it would appear that they have a path towards long-term roles at that level if they continue to progress as players. Plus, from a United perspective, they should be further along in terms of sharpness on the ball than their teammates. Not only did they start training earlier, but they've been facing higher-level opposition on a regular basis.
However, a negative did emerge the day before the USMNT's 2-0 victory over Panama: Birnbaum has suffered what club and country both are calling a "minor" MCL sprain. Talking to Steve Goff at The Washington Post, Ben Olsen referred to it as "very, very minor," which is reassuring. United fans probably associate "MCL sprain" with Dwayne De Rosario being unavailable for over two months in 2012, but thankfully this appears to be a much less severe injury. However, Olsen did add that United's training staff still has to evaluate Birnbaum (who will meet the team in Austin, TX), so we'll see if the prognoses from two different medical teams match.
Assuming the 10-14 day recovery time the USMNT docs expected is correct, it puts Birnbaum's participation against LD Alajuelense in question. That's not to say that he won't be fit to play both legs, but it only leaves him one preseason game - February 20th against FC Dallas - to get some match practice in with the Black-and-Red. On the other hand, with more than a little evidence pointing to the USMNT training too often and too intensely, perhaps the time spent resting will have long-term benefits for Birnbaum's hamstrings and quads.
If Birnbaum isn't fit to start in Costa Rica, the job will almost certainly go to Kofi Opare. The former LA Galaxy man hasn't appeared for United in league play, but he was a regular in the CCL group stage. However, he may have competition if Rauwshan McKenzie - who played well at center back against Malmo FF - is signed in the next couple of weeks.