And there have been more growing pains this season.
Olivier Giroud provided a welcome focal point for Chelsea in their 2-1 win over Crystal Palace - giving Blues boss Antonio Conte a hard decision to make as he plans his team for Wednesday's Champions League clash against Barcelona.
Despite that, Rashford was still very much at the front-and-centre of a Man Utd dominated first half, and people watching only could only sit back in awe at his performance.
This season, the England global has now scored six times in the league, but he has mostly been used as a substitute. "It is just another experience", the England worldwide said.
Former England winger Chris Waddle agreed with Southgate's views, telling the BBC: "Southgate is right, Rashford is a must for the World Cup".
Phil Neville said before the game that there comes a point in a youngster's career when the player needs to start performing against the big clubs. Quite simply, he is going through the same ups and downs that the likes of Ryan Giggs, Cristiano Ronaldo and Wayne Rooney endured before him at United. Marcus Rashford's career had taken a halt after a bright start to the season, but against Liverpool, at Old Trafford is when he chose to bury his poor form.
After the ensuing panic saw £75m man Virgil van Dijk fail to clear as Juan Mata bounded towards goal, Rashford was unmarked to sweep the ball into the rigging for a second goal.
"It was nice that he was here to watch but there are a lot of players in contention", said Rashford.
Man Utd ace Pogba shows off ridiculous handshakes with Rashford and Lingard
I have always said that Romelu Lukaku will get 20 goals for Manchester United. "But you can't cross the line and come on the pitch", West Ham manager David Moyes said. His long-ball tactics proved very effective against a team that presses high with vulnerabilities in defence.
He is clearly growing in confidence as a United player, undoubtedly helped by the fact he knows that José Mourinho rates him very highly.
The last-16 tie is delicately balanced at 0-0 after Mourinho resorted to type by concentrating on obtaining a clean sheet at the Ramon Sanchez Pizjuan three weeks ago.
The result was the first time Jose Mourinho or Klopp have got one over each other on English soil, the three previous league games between the two having been drawn. "Today, we felt Marcus' speed would be important and, with (Anthony) Martial injured.it was a straight choice".
Tottenham and Chelsea took advantage of Liverpool's loss in the battle for the top four with comfortable wins, but Spurs' 4-1 success at Bournemouth was marred by a nasty looking ankle injury suffered by Harry Kane. Of course he was fundamental.
"He was feeling that a little bit in terms of his self-confidence".
"In the situations around the two goals it was not like it should be". When he is fit, his manager will have to decide where - or if - he can accommodate Pogba and Rashford in the same team.
His pace, which put Liverpool's 19-year-old right-back Trent Alexander-Arnold under constant pressure, and work-rate were also evident on Saturday.