Birmingham City manager Harry Redknapp has confirmed Middlesbrough have agreed a deal to sign Ryan Shotton, but the transfer will not go through until cover has been brought to St Andrew's.
"We have agreed terms with Middlesbrough," said Redknapp.
"But he will not be leaving until we have a replacement as we should have four central defenders on the books."
Shotton, 28, made his first appearance of the season as a substitute in Tuesday's goalless draw with Bolton.
He replaced the injured Michael Morrison from the bench, while Cheikh Ndoye also limped off to add to Blues' injury problems.
Morrison, 29, cut his nose while Ndoye, 31, is to have a scan on his foot.
Redknapp could have a selection issue for Friday's game at Burton, with Shotton not expected to feature, although he does have the option of veteran Paul Robinson, who has been on the bench for the past two matches after missing the start of the season through suspension.
"We now have five players out injured," added Redknapp. "And we go to Burton with Ryan Shotton's wife expecting a baby.
"We could have only one [available] centre-half on our books, which is not a good position to be in at this stage of the season.
"This is why we need to get in some quality players if we are going to be a team challenging for promotion."