The Red Devils are in second place in the Premier League table and four points ahead of Liverpool FC in the race to finish as runners up to Manchester City.
Manchester United are a staggering 16 points adrift of Pep Guardiola’s side to highlight the gap between the two derby rivals.
Liverpool FC look to be making steady progression under German head coach Jurgen Klopp, while Maurico Pochettino has transformed Tottenham Hotspur into a strong team.
Manchester United signed Alexis Sanchez in a swap deal from Arsenal in the January transfer window but the Chile international has struggled badly since his move.
Former Spurs manager Redknapp believes Mourinho will find it difficult to bring in world-class signings in the summer transfer window.
“Mourinho knows that he needs to keep improving this team if they’re going to be back to anywhere near what they have been in recent years,” Redknapp told BT Sport.
“He’s got a lot of work to do but he knows that. He can only do what he can do with what he’s got. People keep saying that you’ve got to go and buy players.
“Where are the world class players to go and buy who are going to make the difference? It’s not easy, everyone at the top wants to get them.”
Manchester United haven’t won the Premier League title since 2013 when Sir Alex Ferguson got his hands on his top-flight crown.
The Red Devils won the Europa League and the League Cup in Mourinho’s first season in charge at Old Trafford.
The 20-time English champions will take on Tottenham Hotspur in the FA Cup semi-final at Wembley.
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