NYCFC (12-11-8, 44 points) moved up from eighth to fifth in the crowded Eastern Conference race. It emphatically snapped a six-game winless skid that saw them score twice overall and none in a club-record 509 minutes before Gudmundur Thorarinsson scored on a free kick in the 90th minute of a 1-1 draw at Atlanta on Wednesday.
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Castellanos scored his 14th and 15th goals of the season in NYCFC's flurry in the opening minutes of its first appearance at Yankee Stadium since Sept. 25. He gave NYCFC a 3-0 lead by capitalizing on miscues by D.C. in the third and 11th minutes.
Castellanos got his first goal when United goalkeeper Bill Hamid rushed out of the goal area to play the ball. Instead, Hamid kicked the ball to Castellanos, whose right-footed shot from well outside the box easily reached the back of the vacated net.
Castellanos scored his second goal when D.C defender Steven Birnbaum slipped at midfield while attempting to get a pass from a teammate. Castellanos capitalized when he sped past Antonio Alfaro and lifted a right-footed shot past a sprawling Hamid.
Thiago Andrade scored in the opening minute and Maxi Moralez scored right before halftime by chipping in a rebound of a Castellanos bicycle kick that nearly went in.
D.C (12-14-5, 41 points) dropped into ninth and continued to struggle on the road. The United saw their road winless skid reach six games (0-5-1) and dropped to 2-10-4 away from home this season.
NYCFC goalkeeper Sean Johnson made three saves for his eighth clean sheet. His best saves came in the 22nd minute when he made a leaping save on a header by MLS goal leader Ola Kamara and another leaping stop on Chris Odoi-Atsem in the 66th minute.
Hamid allowed six goals on 15 shots.