Sven-Goran Eriksson admits he would not be surprised if Andreas Isaksson asks to leave Manchester City after being told Joe Hart is number one.
After switching the two men around over the past month, Eriksson vowed to make a final decision on his first-choice in the aftermath of Tuesday's Carling Cup defeat by Tottenham.
Although Hart was beaten twice by Spurs at Eastlands, Eriksson saw enough ability in the England Under-21 star both this week and in his eight previous first-team appearances for the Blues this term to place his trust in the former Shrewsbury man.
It was planned for Castillo to make his debut in the January 2 meeting with Newcastle at St James' Park.
Instead, red tape could delay his official signing by 24 hours, meaning he would not be able to play until the following weekend's FA Cup tie at West Ham.
Meanwhile, Eriksson has confirmed young midfielder Michael Johnson is unlikely to figure against Aston Villa on Saturday.
The England Under-21 international has reported some discomfort from the abdominal surgery he had last month and while Eriksson is confident Johnson will not require another operation, the player has been advised to rest for a couple of days.
