Rashford dazzled as England defeated Costa Rica 2-0 in a pre-World Cup fixture in Leeds on Thursday – the 20-year-old capping his performance with a stunning 25-yard strike.
The 2017-18 season was a struggle for Rashford, who was mostly restricted to substitute appearances under the conservative approach of United manager Jose Mourinho.
However, former United forward Welbeck – now playing for Premier League rivals Arsenal – believes Rashford is benefiting from freedom with the national team.
"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," Welbeck said.
"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 tonight 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."
Welbeck added: "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."