Kieran Djilali has left Fratton Park.
The 21-year-old midfielder has come to a mutual agreement with Pompey to terminate his one-month deal early.
Blues boss Michael Appleton said: “Kieran picked up a lot of niggly injuries in his time here. He’s a talented kid, but we can only work with 20 players and I need to get a defender in.
“I wanted to be straight with him and do it now, so he’s got the chance to find another football club.
“We’re blessed in the wide areas with talented players like Liam Walker, Ashley Harris, Lee Williamson and Jack Compton.
“Kieran would probably end up being a bit of a luxury rather than someone we need, but we wish him all the best for the future.”
Djilali made one appearance for the Blues during his short spell on the south coast, playing 89 minutes of the opening day draw with Bournemouth at Fratton Park.
The Londoner came through the ranks at Crystal Palace and also had spells at Chesterfield and AFC Wimbledon.