"I always wanted to keep him," said Redknapp.
"He was looking for a two-year contract and I said one year and an extra year if he played 20 games was a fair deal for him.
"So far he's played nearly every one and scored three goals.
"He's the key for us, a special player. You don't play for AC Milan, Ajax and Arsenal if you're not a top striker. You should see him in training - he can do things with a ball that are just amazing."