United officials said they did not want to comment right away and are planning to share additional details and a timetable for recovery Wednesday. Fisher, 26, has seen team doctors and specialists since suffering the injury.

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“I will be back stronger,” he said.

His absence is a setback for surging United, which has benefited from his swift, overlapping runs. Projected at the start of the season as a reserve and spot starter, Fisher was thrust into the lineup after the incumbent outside backs, Taylor Kemp and Nick DeLeon, were sidelined for extended time.

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Kemp (hip) will end up missing the entire season, while DeLeon (knee) is close to returning from a four-month layoff.

Fisher, in the last year of his contract, has started 20 of 29 league matches, two more than his previous three seasons combined with the Seattle Sounders. Between May 19 and Aug. 29, he made 16 consecutive starts. He scored against the Portland Timbers on Aug. 15 -- his first goal since college -- and assisted Wayne Rooney’s goal against the New York Red Bulls on Sept. 16.

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Early in the second half Saturday, in his second start since recovering from a minor hamstring ailment, Fisher made a poor back pass on a header and scrambled to recover before Montreal’s Ignacio Piatti could capitalize. As Fisher retreated, goalkeeper Bill Hamid slid into him.

Fisher received treatment on the field and tried to continue but left for good moments later and went directly to the locker room for further evaluation.

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With Fisher sidelined, Coach Ben Olsen has four options: Kevin Ellis, an eighth-year pro who signed before the roster freeze last month and has made two appearances as a sub; Jalen Robinson (four starts this year); Paul Arriola, a midfielder who recently provided back-line coverage; and DeLeon, who started the first 10 matches at outside back before injuring a knee at Los Angeles FC.

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Moving Arriola would risk disrupting an attack that has generated 27 goals in 11 games at Audi Field. DeLeon still must regain match fitness after sitting out all summer, but even before Fisher’s injury, Olsen had suggested DeLeon would figure into his late-season plans, in one way or another.

Chris Odoi-Atsem, a second-year pro from the University of Maryland, is a natural right back, but he has been sidelined much of the year with a variety of ailments.

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With five games left, United (10-11-8) is unbeaten in five straight and two points behind Montreal for the sixth and final playoff berth in the Eastern Conference. D.C. will host the Chicago Fire (8-16-7) this Sunday afternoon.