He also doubts whether big-name players will want to join the club in January due to their current state.
Redknapp told Mirror: "Manchester United gave it to David Moyes who did wonderful at Everton . Then they gave it to Louis van Gaal who is one of the world's great managers. Then they gave it to Jose Mourinho who has won everything there is to win. They couldn't turn it around. And now Ole is struggling with it.
"I think Ole has an impossible job. As I said, Van Gaal had it, Mourinho had it, Moyes had it. They are all top managers. None of them could turn it around.
"People say 'go and get five or six players'. But where are they going to get them from? Where are they? Who are they playing for? Are they going to move to Man United?
"You're not going to get Harry Kane. They're not going to go and get Raheem Sterling. Where are they going to find them from? Easy to say they need them but where are they going to find them?
"It's easier said than done to build a new team. It's a very long-term job they have on their hands."