Morrison is joining QPR to the end of the season.
"You need people who can open the door, someone who can unlock a defence," Redknapp told Sky Sports News. "I've always had someone in my team who can change a game.
"I've always enjoyed having someone like that, whether it was Paolo Di Canio or Paul Merson when I went to Portsmouth, someone who can do something special and turn a game for you.
"I just think Ravel is one of those lads who can do that for us. I'm not sitting here saying it's not a gamble because he is a gamble.
"If he didn't have a problem somewhere, he wouldn't be coming to QPR. He would be still at United because he's got that talent.
"It's a chance you take sometimes in life; you have to take a gamble. Sometimes they come off, sometimes they don't - but if you don't buy a ticket, you are never going to win the lottery."
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