A team official said doctors removed bone fragments, which they believe were the source of the pain.
Training camps typically open in mid to late January, six weeks before the openers.
Birnbaum, who will turn 30 on Jan. 23, has appeared in 173 regular season matches (171 starts) and is United’s longest-serving player without interruption.
Birnbaum and Frédéric Brillant, 35, have been United’s first-choice duo in central defense for much of the past three years. Late last season, however, second-year pro Donovan Pines earned regular starts in place of Birnbaum or Brillant.
United is in the market for a fourth center back. One possibility is former D.C. academy player Michael Edwards, 20, whose contract with Wolfsburg expires this summer. Edwards plays for Wolfsburg’s second team in the German fourth division.
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