The Red Devils bounced back to form with a 3-0 win over Everton on Saturday, ending a poor run of results at Goodison Park.
And Redknapp thinks the win came down to one thing - the inclusion of Chris Smalling and Phil Jones at the heart of defence.
Louis van Gaal played Daley Blind and Ashley Young in the previous game, a 3-0 defeat to Arsenal.
And former Liverpool midfielder Redknapp thinks it was no coincidence that with two natural defenders at the back United did better.
"Against Arsenal Louis van Gaal got it badly wrong," Redknapp told the Daily Mail.
"Ashley Young and Daley Blind are not natural defenders. If they had played on Saturday, Romelu Lukaku would have eaten them up.
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"But Van Gaal realised his mistake and went back to Chris Smalling and Phil Jones. They relished the battle. And with Marcus Rojo and Matteo Darmian, Van Gaal had a proper defensive unit.
"With the money they've spent and the players they have, United are more than capable of winning the league. And with the Manchester derby up next week, they have to build that challenge around a solid defence."