When D.C. United kicks off against FC Dallas late Saturday afternoon at RFK Stadium, starting defender Steve Birnbaum will be on his way to Ohio from Guatemala and reserve forward Alvaro Saborio will be bound to Costa Rica from Jamaica.  Seven Dallas players can be found scattered about, with the first-choice goalkeeper in Portugal.  The soccer orbit is in the grips of an official FIFA window, when most leagues go dark as many players join national teams for World Cup qualifiers, friendlies and other formal competitions. MLS is an exception. The league did not schedule a full slate of matches this weekend — only three of a possible 10 — but the domestic activities during an otherwise dormant global period have left six coaches without their full arsenal of players.  In defense of assigning games during the FIFA calendar, MLS cites scheduling challenges over the course of the year. Fewer weekend matches means more weeknight dates, which clutter the weekly routine and traditionally draw smaller crowds in most markets.  Over the years, MLS has become more compliant. It takes time off for the World Cup every four years. This season, the league will take a 16-day break in June during Copa America, the prestigious South American competition staged in the United States for the first time. MLS also lightened the schedule during FIFA windows in late summer and October, and didn’t plan any playoff games during the FIFA period in November.  With Birnbaum on U.S. assignment for two qualifiers against Guatemala, United Coach Ben Olsen is likely to partner Kofi Opare with captain Bobby Boswell in central defense. Saborio’s absence for Costa Rica’s qualifiers against Jamaica thins attacking depth but does not impact the starting lineup.  Olsen’s choices are easy compared to his counterpart’s, Oscar Pareja.  Dallas, the preseason Western Conference favorite, is down three sure starters: goalkeeper Jesse Gonzalez (Mexican under-23 squad), defenders Walker Zimmerman (U.S. U-23s) and Maynor Figueroa (Honduras national team). It is also without two probable starters: midfielder Kellyn Acosta (U.S. U-23s) and defender Atiba Harris (St. Kitts and Nevis national team).  In addition, reserve forward Tesho Akindele is on Canadian duty and Moises Hernandez is with Guatemala, though, under normal circumstances, the latter probably would not have accompanied the Dallas team to Washington anyway.  United will look to exploit Dallas’s numerous defensive changes, but as Olsen cautioned, “their top attackers remain the same. And it might be the most dynamic attack in the league. Their flair and pace will not change.”  Among the other four teams in action Saturday, the Vancouver Whitecaps were hit hardest (six absences, including four regulars). Their opponent, the Houston Dynamo, lost two players. New York City FC (three excused players) and the New England Revolution (two) will meet in the other match.  MLS does allow teams to request breaks during specific FIFA windows. United, for instance, received a bye in early October.  “At least we have some say in the matter,” Olsen said.  United is still seeking its first victory in 2016: Aside from an 0-1-2 start in MLS, D.C. recorded a defeat and draw in elimination from the CONCACAF Champions League quarterfinals by Mexico’s Queretaro.  “We’re itching for it,” Boswell said. “We need a win.”  Over five matches, United has scored scored three goals: rookie Julian Buescher’s sensational strike late in the Champions League’s second leg, Lamar Neagle’s clinical finish early in the league opener at Los Angeles and Fabian Espindola’s late equalizer in the home opener last weekend against the Colorado Rapids.  With several candidates in the mix, Olsen continues searching for the most effective attacking unit.  “It’s going to take at least a few more games to figure out,” midfielder-forward Chris Rolfe said. “Ben has got to get a proper look at everyone.”  United notes: Finnish midfielder Markus Halsti, sidelined for several weeks with a sprained knee, has resumed personal workouts but has yet to rejoin training. …  Rookie midfielder Paul Clowes reported to United’s third-division affiliates, the Richmond Kickers, for the USL opener Saturday against the Harrisburg City Islanders. Another candidate for assignment, midfielder Collin Martin, is out with a foot injury. Defender Jalen Robinson remained in Washington in order to provide depth in Birnbaum’s absence. …  Forward Andrea Mancini, who has agreed to contract terms, is in Italy awaiting a work visa. He had been with the club at training camp. Mancini, 23, is a former Manchester City prospect most recently aligned with Hungarian clubs. …  Dave Johnson and former U.S. national team forward Bruce Murray will handle announcing duties for ABC7’s first United match under the new local rights agreement between Sinclair Broadcast Group and the team.  Murray also provided color commentary for the March 12 game at New England, carried by Sinclair’s NewsChannel 8. United has yet to formally name a full-time analyst to join Johnson, who is handling play-by-play duties for the 21st season. ABC7 will carry five of the next six matches and seven overall this year.  Sinclair-owned stations in Baltimore, Richmond, Norfolk and Roanoke will show Saturday’s match.  D.C. United vs. FC Dallas  When: Saturday at 5:30 p.m. ET.  Records: United 0-1-2, 2 points; Dallas 2-1-0, 6 points.  D.C. probable starters: GK Travis Worra; Ds Sean Franklin, Bobby Boswell, Kofi Opare, Taylor Kemp; MFs Patrick Nyarko, Marcelo Sarvas, Nick DeLeon, Chris Rolfe; Fs Luciano Acosta, Fabian Espindola.  Dallas probable starters: GK Chris Seitz; Ds Ryan Hollingshead, Matt Hedges, Aaron Guillen, Zach Loyd; MFs Michael Barrios, Carlos Gruezo, Victor Ulloa, Mauro Diaz, Fabian Castillo; F Maxi Urruti.  Steven Goff is The Post’s soccer writer. His beats include D.C. United, MLS and the international game, as well as local college basketball.
With MLS matches scheduled during the FIFA window, D.C.'s Steve Birnbaum and Alvaro Saborio, as well as seven Dallas players, are unavailable.