Rashford came on as a second half substitute for the Red Devils and was in the right place at the right time to tap home Wayne Rooney's cross in the second minute of injury time to give Jose Mourinho's side three wins from three games in the Premier League.
Overall, though, what was learnt from the game?
Rashford makes his mark
Rashford has had to wait to show Mourinho what he can so far this season but when the team needed him he stepped up and got the crucial winner.
The striker had an amazing impact when he came into the team last season but it was always going to be hard for him to start the season with the likes of Zlatan Ibrahimovic and Rooney going to start.
When the team needed him though against Hull he came on the pitch and from the first minute showed the desire and looked like the one that was going to do it for United and that he what he did. The young striker has now made it a real difficult decision for Mourinho to not pick him after the International break after his brilliant performance.
Mkhitaryan must start the Derby
Another player that had to wait to make his presence known in the team was Henrikh Mkhitaryan. The Armenian has had to settle for a place on the bench for the first few games this season but when the team needed his guile to break through a difficult Hull defence he delivered and must start against Manchester City after the International break.
The United players struggled throughout the game to get a goal but when Mkhitaryan came on the pitch things changed as he immediately started running at the Hull players with his extra pace when they were tired.
The playmaker showed his class and what he can do for the team and he must start in either the number 10 role or out in the right wing in the next match as he can be the man that can get the decisive moment in the game that gets the team the win.
The team showed the desire to get the victory
There were signs of the 'old United' against Hull as the team kept going until the final whistle to get the winner. The team poured forward in numbers to break down a stubborn defence and their persistence paid off with a deserved winner.
Every single player on the pitch never gave up on the day which was a very important thing see from the fans perspective as they can now believe in this team that they are going to get the win even in difficult situations.
'Fergie Time' is something that many United fans are used too and to score the winner so late on yesterday made it very special as it brings back so many good memories that Mourinho is starting to bring back to the club after a few years without it.
Blind deserves credit
As the season began with Daley Blind in defence alongside Eric Bailly, many people questioned Mourinho as to why he did that but so far the pair are working so well together.
Blind is the man at the back that keeps everyone calm which in turn is making Bailly play so much better. The Dutchman is such an underrated player for the club and he deserves so much credit for showing he can play anywhere the boss plays him.
Against Hull against two in-form strikers, Blind kept them quiet with David de Gea not having anything major to do which just shows how good a player he is. Chris Smalling and Phil Jones will struggle to get game time if he keeps up his fantastic form and lets hope he does as he deserves every single bit of praise that he gets.