The former Tottenham boss has taken over a side rooted to the foot of the table, with just four points from 13 games and without a win all season.
He has saved Portsmouth from relegation and took a Tottenham side from bottom of the table to the Champions League the following season.
Be he said: "We mustn't kid ourselves, this is much tougher that those two jobs in my opinion, but I'm optimistic we can pull it off.
"I want to be successful here, really badly. I want to keep this team in the Premier League.
"If the players try as hard as I am going to try we'll be successful."
Redknapp confirmed a deal was being done to bring in long-time assistants Joe Jordan and Kevin Bond to assist him.
And he warned the players he would not tolerate a lack of effort.
"We've got to do better to give ourselves any chance of getting out of this mess," he said.
"I need 11 people with ability to put effort in. If they don't they're not going to play and I'll get 11 people who do. I've got no time for people who lose the ball and stand their with hands in the air."
On the team spirit, he added: "The lads I met yesterday were all upbeat.
"They've got to realise they are in this position because they haven't really performed to their capabilities and they've got to do better.
"The players have to take responsibility for performances, they've got to up their performance level and get us out of trouble."