Kanu, 30, is considering fresh terms with Pompey but has not returned to pre-season training, and La Liga clubs including Atletico Madrid are on alert while the deal is in the balance.
"If I get two strikers in, suddenly things look different," said Redknapp.
"Suddenly he's got to make his mind up.
"So if Kanu doesn't take the offer in the next week or so, it might not be there. That's the problem for him."