Rashford made it onto yet another Mata through ball before threading it past goalkeeper Jon Kempin
All the noise has been around Romelu Lukaku here in Los Angeles, and not always with the approval of his neighbours who resorted to calling the police on Manchester United's new £75million signing.
But Marcus Rashford provided a timely reminder that there is more than one striker at Old Trafford capable of leading the line and scoring goals for Jose Mourinho's team this season.
Mourinho suggested in the build-up to United's first game on tour that he needed 'a target man, a clear No.9, which is not Rashford'. The teenager responded by scoring twice inside the opening 20 minutes as United breezed past a poor LA Galaxy at the StubHub Center in Carson.
United's players celebrate following Rashford's opener which came inside two minutes against LA Galaxy
Antonio Valencia and Marouane Fellaini celebrate after Belgian's strike gave United a comfortable 3-0 lead
Henrikh Mkhitaryan (left) celebrates with new team-mate Romelu Lukaku after scoring United's fourth goal
Former Chelsea full back Ashley Cole (right) catches Lukaku late with a challenge as he tries to stop the striker
Lukaku - wearing United red for the first time - looked a little off the pace during the 45 minutes he played
Juan Mata (left) was running the show from midfield and will have been pleased with his first half contribution
Paul Pogba (left) and Romelu Lukaku warm up at half-time before coming on in the second half for United
Former Manchester United and LA Galaxy man David Beckham was in attendance for the International Champions Cup match
Jose Mourinho catches up with former Chelsea left back Ashley Cole ahead of the match against LA Galaxy
'It is not going to be our main system. We are always a team that normally plays four at the back, but during the season we maybe need to do it.
'Last season we did need to do it against Rostov at home and away, so maybe we need to adapt to some matches and pre-season is exactly for this – especially the second pre-season, which it is now.
'We are going to try what we have been doing in training in these first few days.'
As it was, United were hardly tested by a below-strength LA Galaxy team struggling midway through the MLS season.
Mourinho had talked about the challenge of facing players with more intensity than his team, but it was United who looked sharper as they surged into a three-goal lead inside half an hour.
Twice in the opening 20 minutes, Rashford benefitted from carelessness in the home ranks. He struck first after two minutes when Jesse Lingard dispossessed Nathan Smith and the ball ran kindly for Rashford. The youngster seized on his opportunity to go through on goal and beat goalkeeper Jon Kempin with an unerring finish into the bottom corner with his right foot.
It was a similar story for the second goal. This time Ander Herrera was the man to steal the ball from the home side and slip it to Juan Mata. The Spaniard's pass released Rashford again and the execution was just as precise as he slid the ball into the bottom corner again.
The shirts of Romelu Lukaku and Paul Pogba hung up in the United dressing room
Rashford should have completed his hat-trick moments before he was replaced at half-time as Michael Carrick sent him clear from halfway with a wonderful pass. Rashford was onside and raced away with just Kempin to beat, but this time the Galaxy keeper stood string and made a fine save to deny him.
By then, United were three up after Marouane Fellaini had got his name on the scoresheet. Lingard and Herrera were involved again, playing a one-two to allow Lingard to get to the byeline. He showed presence of mind to wait and pick a pass to Fellaini who swept the ball home with his left foot from 15 yards.
Chris Smalling also had an effort ruled out for offside and Mata's clever chip from the edge of the box forced Kempin into a fine save as United threatened to repeat their 7-0 win over the Galaxy in 2014. That match was played in front of 86,500 fans at the Rosebowl, but it has to be said there were a few empty seats in the 27,000-capacity stadium here in Carson.
Changing the entire line-up at half-time interrupted United's flow somewhat. However, they should still have added to their on several occasions before Henrikh Mkhitaryan scored the fourth as both Lukaku and Lindelof missed chances, and substitute keeper Rowe made several excellent saves.
He was beaten in the 66th minute when the Galaxy were punished for another sloppy mistake in midfield. Mkhitaryan stole possession from Jermaine Jones and raced away before exchanging passes with Anthony Martial. Rowe came out to stop him but never stood a chance as the Armenian slipped the ball past him.
Martial made it five in the 72nd minute, sweeping home from just inside the box following a fine run and pull-back by Timothy Fosu-Mensah, but it was the Galaxy who ended the game with their tails up.A sweeping move upfield that was completely out of context with anything that had preceded it from the MLS side ended with former Tottenham midfielder Giovani dos Santos curling the ball beyond Joel Pereira.Then Lindelof failed to get off the ground in the 88th minute as Dave Romney rose to meet a corner with a header that appeared to take a touch off Dos Santos on its way into the net.
The StubHub Center, with a capacity of 25,000, played host to United v LA Galaxy
United supporters gather ahead of the game at the StubHub Center in Carson in California