Redknapp told the Scottish Sun he is keen to bring Petrov to Portsmouth so he can play in England.
He said: ‘We’re really interested in him but they don’t want to sell him.
‘I’ve been told he’s put in a transfer request and he’s a player I rate very highly. Personally, I’d let him go if I was Celtic boss.
‘Sometimes you might not get the best out of a player if he doesn’t want to be at the club.
‘If he’s handed in a transfer request then he can’t be happy.
‘But he belongs to Celtic and I don’t want to get into a war with them.
‘If they decided they want to do some business with us then we would definitely be interested.
‘Celtic are a fantastic football club but it’s not a fantastic league. The Premiership is a million miles in front.’