It has been reported by The Northern Echo that the Blues and Boro have agreed on a fee of £2m for the 28-year-old, but Redknapp has indicated that the transfer will not be finalised until a replacement is found.
The Blues boss told reporters: "He's a good lad but Middlesbrough want him. He also fancies the move so if he goes it leaves us with only two defenders and we need four.
"We've agreed a fee and we're just waiting until we get someone in now."
Shotton will leave Birmingham having scored three goals in 54 appearances for the West Midlands outfit in all competitions.
Boro manager Garry Monk has already made eight first-team signings since arriving at the Riverside Stadium.