Rooney was left out of United's squad for the 2-0 Community Shield win over Wigan Athletic yesterday, but will report for England duty today as rumours persist over his future.
Redknapp thinks that Rooney should force United manager David Moyes to sell by handing in a transfer request if he is serious about a move away from Old Trafford this summer.
He told the Daily Mail: "Wayne Rooney needs to leave Manchester United. It's the best thing for him but, by making Chelsea wait, United are casting a shadow across their own season.
"David Moyes doesn't seem to want to be the manager who sells the best player of his generation twice, so Rooney may have to force the move by asking for a transfer. There is always a way back but the best thing for him is to play for Jose Mourinho.
"Right now, he is injured for his club but will play for his country this week. It can't go on and, with two years left on his contract, this is the best chance to cash in. I expect the deal to happen before the deadline."
United boss Moyes has been adamant that Rooney is not for sale since taking charge of the Red Devils, swiftly rejecting a pair of bids from Chelsea in recent weeks.