(D.C. United photo) (D.C. United photo)

D.C. United forward Conor Doyle, a key reserve on MLS’s most surprising team, will miss the rest of the season with a knee injury, the club announced Tuesday.

He had surgery Monday to repair torn meniscus in his right knee, an injury suffered in training last week. Doyle, 22, is expected to be sidelined four to six months.

In his second season in Washington after three in England, the Texas native has appeared in 17 of 21 league matches. He started just once and did not score but provided late-game relief on the frontline. Last year, after arriving midseason from Derby County, he scored twice in 14 appearances.

Doyle is the second back-up to suffer a season-ending injury in recent weeks. Andrew Dykstra, the second-choice goalkeeper who made four starts this year, ruptured an Achilles’ tendon in a friendly against English club Fulham. Starters Chris Pontius (hamstring) and Jeff Parke (migraines) are also recovering from long-term ailments. Pontius has missed the entire season but might return in the fall; Parke’s return is uncertain.

Doyle’s prognosis may accelerate United’s plans to bolster the squad immediately. Even before that injury, club officials said they were seeking help at several positions. While MLS’s international transfer window will close Wednesday, clubs are allowed to sign free agents and make trades until Sept. 15.

With Doyle unavailable Sunday in Houston and others out with minor injuries, United’s two substitutions were inexperienced homegrown players, midfielder Collin Martin, 19, and forward Michael Seaton, 18.

One year after winning three of 32 matches, United (11-6-4) is second in the Eastern Conference and third in the overall points race. The club will visit Real Salt Lake on Saturday night.