In his column for The Sun, Redknapp wrote: "Tottenham's loss is certainly our gain at Portsmouth. And he jumped at the chance to come here.
"He knows David James, he knows Glen Johnson very well and he knows me. He was still just a boy when I left West Ham but to have a second crack of the whip with him makes me very excited.
"The fact his old club are propping up the Premier League will make no difference. He will want to score against them. There is no emotion, no sympathy.
"He thrives on goals, he feeds on confidence and I reckon he could end up scoring 20 goals for us this season."