The Red Devils signed the Chile international in a swap deal with Arsenal that saw Henrikh Mkhitaryan move to the Emirates Stadium in the January transfer window.
Sanchez established himself as one of the top players in the Premier League during his three and a half seasons at the north London club.
The 29-year-old forward scored 80 times in 165 games in all competitions for the Gunners, winning two FA Cup titles and the Community Shield.
The South American made two assists in his Manchester United debut to help Jose Mourinho’s side ease to 4-0 win against Yeovil Town in the FA Cup fourth round last week.
Sanchez was powerless to prevent the Red Devils from losing 2-0 to Tottenham Hotspur at Wembley on Wednesday night to fall 15 points behind leaders Manchester City.
Former Spurs boss Redknapp is confident Sanchez can have a positive impact at the Red Devils following his January switch.
“He’s a world-class player. He is top, top drawer. It’s an amazing signing for Man United,” Redknapp told BT Sport Premier League Tonight on Wednesday night.
“He can take them back to the level they’ve been at for so many years, challenging for and winning the league in the next two or three years again.”
Sanchez scored 60 times in 122 games in the Premier League during his mixed spell at Arsenal.
Manchester United will take on Huddersfield Town in their next top-flight game at Old Trafford on Saturday afternoon.
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