The former midfielder has also advised striker Christian Benteke to target Daley Blind, rather than get into a physical battle with Chris Smalling.
The two sides lock horns again this weekend looking to return to winning ways after suffering demoralising defeats before the international break.
Liverpool were beaten 3-0 by West Ham United at Anfield, while Louis Van Gaal’s side were beaten 2-1 by Swansea City, meaning both sides are keen to make an impression when old rivalries are rekindled at Old Trafford.
Brendan Rodgers will be without star midfielder Philippe Coutinho for the game, and Redknapp believes this may force him into packing the midfield.
“Liverpool will be without their chief creator, Philippe Coutinho, who is suspended after being sent off against West Ham,” he told Sky Sports.
“It's a huge loss, the one player they wouldn't want to be without for a game like this. Coutinho's absence, though, may force Rodgers' hand to play an extra midfielder and set Liverpool up to be hard to beat, solid and tough to break down.”
Get crosses into Benteke
The former Reds midfielder has also urged his old side to play to Christian Benteke’s strengths, by putting plenty of crosses into the box.
Liverpool have so far struggled to make the most of the Belgian striker's talents since his move from Aston Villa this summer, with their game working in direct contrast to the one that made him such a hit at Villa Park.
And while Redknapp has urged Liverpool to put over more deliveries, he has also advised Benteke to target United defender Daley Blind - as he is likely to get more joy out of tussles with him than he is with Chris Smalling.
“If they do that, it's crucial they play to Christian Benteke's strengths - and United's weakness - by putting plenty of crosses into the box,” he added.
“Benteke shouldn't entertain the thought of getting into a scrap with Chris Smalling on Saturday, he should be pulling on to Blind every time Liverpool get the ball in wide areas - and Liverpool need to get crosses into him.”