The Senegal international was signed in a £30m deal from their Premier League rivals in the summer transfer window as Jurgen Klopp added more attacking prowess to his Reds squad.
Mane has proven a big hit with the Liverpool supporters after a return of three goals and one assist in six Premier League appearances for the 18-time English champions.
Liverpool are currently in fourth place in the Premier League table and two points behind leaders Manchester City after a run of four successive top-flight victories.
Former Reds captain Thompson believes Mane holds the key for Klopp against United ahead of the visit of Jose Mourinho’s side at Anfield on Monday night.
“I think the big one is going to be Mane, mainly because of his pace against Manchester United’s back line. He’s the one player really that I think can exploit them,” Thompson told Sky Sports.
“Marcus Rashford is obviously quick and has tremendous potential but I think with Mane’s experience against United’s back line he will cause them tremendous problems, when there’s a turnover of position is when he really comes into his own.”
Liverpool were 2-1 winners against Swansea City at the Liberty Stadium in their last Premier League game ahead of the international break.
United are in sixth place in the Premier League table and six points behind Pep Guardiola’s City.
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