That Birnbaum is generating this interest should not be considered a huge surprise. Since being drafted in 2014, Birnbaum was a Rookie of the Year finalist, and his 2015 season ended with an invite to the United States National Team camp this January (he scored the game-winner against Iceland), and appeared in the USA's qualifier with Guatemala a month ago yesterday.
If he leaves anytime soon, the 25-year old Birnbaum would be the second D.C. player to leave for a European side recently, after Perry Kitchen's contract expired last year, he signed with the Scotland side Heart of Midlothian F.C. He would also be the second young American center back to leave for Europe after former New York Red Bulls player Matt Miazga was signed by England Premier League side Chelsea. Birnbaum, listed as making $96,000 guaranteed compensation in 2015, would almost certainly receive an increase, and D.C. would receive a fee from the transfer, should a deal go through.
Were Birnbaum to depart this summer, United would undoubtedly turn to Kofi Opare to take over as a starting center back. The former LA Galaxy defender has mostly acquitted himself well since coming over in 2014. However, in terms of established depth, Opare is it for United. Markus Halsti has played center back in the past, but it's not a natural fit for the Finn, who may or may not be around past the summer window anyway. Homegrown product Jalen Robinson is probably most comfortable as a center back, but it's rare to see a 5'9" center back succeed in MLS.
More on this news as it develops.