United signed Sanchez in the January transfer window as part of a swap deal with Arsenal which saw Henrikh Mkhitaryan head to the Emirates.
England international Rashford has shown moments of class to justify his inclusion in the United first team this season, but has had a frustrating time in front of goal.
The 20-year-old has scored four Premier League goals in the league this season, failing to capitalise on a number of opportunities which Jose Mourinho labelled as “unbelievable chances”.
Former Tottenham and Liverpool midfielder Redknapp is surprised that United went in for Sanchez, as players of the Chilean’s calibre do not come around often for a cut-price deal.
"I don't blame Manchester United for bringing in Alexis Sanchez,” Redknapp told Sky Sports.
“His wages are expensive, but you would normally have to pay around £70million for a player like that in the current market.
"They have dodged the transfer fee and signed a winner who will give them fantastic energy and quality."
On the other hand, Redknapp is worried that Sanchez's arrival could block Rashford’s first-team chances - and with the World Cup in the summer, it could prove damaging for the youngster.
"On the other side of it, though, I can't help but feel disappointed about what it does to a player like Marcus Rashford," said Redknapp.
"Some would say that great players come and go and you've got to get on with it, but the bottom line is that Rashford has now dropped down the pecking order.
"Due to his reputation and the fact he has been bought by the manager, Sanchez will always go ahead of Rashford whether he is playing better or not.
"It's the hardest thing for a young English player. You feel like you are just getting to the top of the pecking order and then suddenly, bang, it's like a sledgehammer hits you.
"Your club have signed a world star. Mourinho has said Sanchez can be an inspiration to Rashford but I don't think that's what he needs right now.
"We're talking about an established England international.
"There is only one way to learn, and it's not by sitting next to someone in the dressing room, it's by playing in the team.”