The Nigerian striker - out of contract this summer - has reached deadlock with Pompey over negotiations for a new deal.
The 30-year-old is demanding a straight two-year contract as reward for scoring 10 Premiership goals last season.
Redknapp told The News: "I think Kanu might move somewhere else.
"It wouldn't be the biggest surprise in the world to me if he did. He's a strong character.
"He's been offered a year and will get another year if he plays 20 games in all competitions. That's the way it works. It's fair.
"The 20-game figure is a safeguard in case the fitness of a player suddenly goes.
"If a player has been injured, for instance, and hasn't played 20 games, but he's done well and is in good shape, then I could look to keep him.
"It's not as if I'm going to bomb someone out if they've done well but haven't played exactly 20 games."