Four dropped points in two games against Sheffield United and Arsenal saw momentum slip from Ole Gunnar Solskjaer’s Premier League title contenders.
But the remorseless demolition of a woeful Saints side hit with two red cards and a nine-goal thrashing could be the spark needed to get United going again.
“Hopefully it can push us all in the right direction for the rest of the season now,” said Rashford, who scored the second of United’s record-equalling haul.
“We’ve dropped a few points in the last few league games so yes, we needed a night like this really, just to get us going again.
“We can’t really ask for much more, so we need to use this performance and the result as motivation really, and something to mirror in the next few games.
“If we do that, we’ll pick up three points.”
Rashford was on the scoresheet early at Old Trafford on Tuesday night and will be looking to help put struggling West Brom away at the Hawthorns on Sunday.
And the striker hopes another win can ignite another unbeaten run like the 13-match Premier League streak which came to an end against the Blades.
“Whenever we can, we want to put these types of results together and go out and really finish teams off and this was a perfect example of that,” said Rashford.
“It’s always good to score lots of goals and it’s three points that we needed.”
He added: “We’ve dropped some points in the last few games and we needed to get back to winning ways so that was a great opportunity to do that.”
Daniel James meanwhile revealed that Solskjaer demanded his United side show no mercy to struggling Southampton at half-time.
The Wales international scored the ninth goal as United ran riot, equalling their best ever Premier League win.
But suggestions United’s manager lacks the killer instinct to succeed were knocked after Solskjaer sent out his men for the second half 4-0 up and demanding no let up from his side.
“It is easy coming in at half-time and thinking we’ve won the game but Harry Maguire and the gaffer especially were on to the boys saying: ‘go out second half and do exactly the same’,” said James.
“Leagues, sometimes, goal difference is massive, but also for our confidence and doing the right things and today was the day we could go and do that.
“The second half takes us into the next game as well and we went and scored more goals.”
United undid the damage of the 6-1 defeat to Spurs at Old Trafford earlier this season and added a further four in the plus column.
James was becoming only the third player to hit a ninth goal for their side in a Premier League match after Andy Cole (from 1995) and Jamie Vardy (2019) but insisted the victory was more about the team.
“We are all confident at the minute, but getting the goals today is great for everyone,” said the winger.
“Everyone is involved in all the goals really, so it was nice to see and nice to go on and get my own goal as well.”