Jose Mourinho brought in Sanchez from Arsenal last month in a deal which saw Henrikh Mkhitaryan move in the opposite direction.
And the Chile international has started every Premier League game so far on the left of United’s front three.
That position has often been occupied by Rashford this season with Anthony Martial also battling for a starting berth on the left.
But the England international has no featured in two of United’s last three fixtures and wasn’t even named in the matchday squad in Sunday’s 1-0 loss to Newcastle, although Jose Mourinho claimed his absence was due to injury.
Rashford has started just 13 league games all season under Mourinho and his form has stagnated since bursting onto the scene two years ago.
And Henry believes Rashford may have to leave in order to find more regular football to continue his development.
“Leave is a strong word because I’m sure being from there he wants to play for United but he has to play,” Henry said on Sky Sports’ Monday Night Football.
“It’s going to become even more difficult for him because Sanchez has arrived.
“I think at the end of the season he’s going to be thinking ‘do I stay?’
“Or you just make it and show everything you can play with all those guys around.
“For me, if you’re good enough you can make it work everywhere you want. Having said that if they can’t allow you to carry on that momentum that you had from that first season it can become difficult.
“What I’m saying to him is ‘you need to play’. It can also be at Manchester United but he hasn’t had the chance to carry on the momentum he had, especially at a young age you play on that sometimes.”