The 19-year-old has carried over his stellar form from last season, scoring on his Champions League debut against Basel last Tuesday. Rashford has excelled whilst playing on the wing this campaign, benefitting off United's new forward signing, Romelu Lukaku.
And although Barnes - who played over 300 games for Liverpool - believes it is the best position for Rashford to develop his immense potential, he is confident the England international can score goals and be effective no matter where he lines up on the pitch.
"It depends on how he develops, of course," said Barnes, who spoke exclusively with Tribal Football.
"He's still a young player with great potential and he seems to do most of his best work coming in from wide positions.
"When he develops in the next couple of years he may end up as a number 9, but it really depends on his development.
"At the moment, Lukaku is playing in that position, so he won't play as a number 9. I don't think necessarily wants to play no. 9 because it's a very difficult position, but long term he may play down the middle, but it doesn't really matter because he will score goals, he'll be effective.
"As to whether he plays wide or down the middle it doesn't really matter, I think that we've too often seen young players like Theo Walcott say I want to try and play down the middle.
"If your a striker, gone are the days when, to be a number 9, you have to be strong, good in the air, good holding the ball up, good with your back to goal.
"Rashford doesn't do that so maybe long term it will be, but at the moment it think playing in the inside left or inside right is his best position."
John Barnes, was speaking to Tribal Football on behalf of Bonus Code Bets.