United officials said they would receive a detailed timetable for recovery after the operation, but it seems certain Mora will miss substantial time.

The injury occurred in the 11th minute. Mora was attempting a slide tackle on Ruan when the Brazilian’s left knee crashed into the right side of the D.C. player’s jaw. He was treated on the field for several minutes as blood spilled from his mouth.

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With a hand to his jaw, Mora walked off the field and retreated to the locker room, then to a hospital for X-rays, which confirmed the fracture.

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Chris McCann, who signed with United from MLS champion Atlanta in preseason, made his D.C. debut and played the final 75 minutes. The 31-year-old Irishman is not as quick or attack-oriented as Mora but brings years of experience on the English circuit (Burnley, Wigan and Coventry City before two years in Atlanta).

Mora, 26, started 28 of 34 matches last season (with two appearances as a substitute) and all four games this year for United, which, at 3-0-1 and 10 points, is off to its best start since 2006. Its regular season unbeaten streak has climbed to 14 matches.

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With the same starting lineup in place for the longest stretch in five years, the defense registered shutouts in the first three games this year before conceding Dom Dwyer’s second-half header.

Wayne Rooney recorded a goal and assist in the first half as United geared up for Saturday’s early-season showdown with league-leading Los Angeles FC (4-0-1, 13 points) at Audi Field.

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McCann is expected to start in Mora’s place, but the injury further depletes United’s depth at the outside back positions. Oniel Fisher, who had ACL knee surgery last fall and suffered subsequent complications, is not expected to return until late in the year. Chris Odoi-Atsem is getting himself back into shape after undergoing treatment for Hodgkin’s lymphoma.

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Rookie Akeem Ward, a left and right back who has been starting for second-division Loudoun United, is a prime candidate to provide cover for McCann.