The midfielder netted three and set up a forth as United ran wild in the first half.
All four of United's goals came in the first 31 minutes of the match. Najar netted 13 minutes in after De Rosario picked him out with a diagonal ball that opened up RSL's back line. Najar provided the cross for De Rosario's first nine minutes later. The Canadian added his second and the team's third five minutes after that.
The best goal of the bunch came in the 31st, a stunning free kick from 35 yards out that caught RSL goalkeeper Nick Rimando off guard.
Álvaro Saborío came on in the second half and pulled one back in the 86th, but United's first half outburst was more than enough to put the match to bed.
De Rosario's second hat-trick of the season vaulted the midfielder to the top of the MLS scoring charts while helping end a five match winning run for RSL.