Welbeck came through the academy at Manchester United before his move to Arsenal in 2014, just as Rashford has done now.
"He was a quite a few years younger than me, but his older brother (Dwaine) knows my older brother, so I knew about Marcus for a very long time," Welbeck said.
"He has a lot of that youthful enthusiasm. But he is so mature around the place as well at times. He knows when to curb his enthusiasm.
"He has been free to play. The manager told him to go out there and be free - express yourself. Marcus has the qualities to have an impact on every game."