The 20-year-old produced a standout performance for the Three Lions in their pre-World Cup friendly win over Costa Rica on Thursday night, scoring a 25-yard stunner for the first of his side's two goals.
Rashford's display was in stark contrast to what supporters became used to in a frustrating campaign for United last time out, with the striker often being shunted out wide to accommodate Romelu Lukaku through the middle.
Speaking after England's victory at Elland Road, Welbeck suggested that a change in mentality is to thank for getting the most out of Rashford.
"I have enjoyed training with Marcus and from playing against him in the Premier League, I know the qualities he possesses and [on Thursday] he was brilliant," he told reporters.
"He has been free to play. The manager told him to go out there and be free – express yourself. The structure we have got as a team means we all know what we have got to do, so when you are in a certain position you know you have got lots of choices.
"He is – how old? – 20. He has got a lot of that youthful enthusiasm. I think he was buzzing all around the pitch. But he is so mature around the place as well at times. He knows when to curb his enthusiasm. Marcus has the qualities to have an impact on every single game.
"There are a lot players in there who can make an impact in any given moment. I think a lot of the players are really looking to going and seeing what effect they can have."