Wigan chairman Dave Whelan claims his club have been granted permission to speak to Brentford boss Uwe Rosler.
Stevan Jovetic's agent insists that the Manchester City striker has settled into life in England, despite exit rumours.
The 29-year-old was named in Birmingham's development squad for their clash with Coventry on Tuesday.
Castelen is available on a free transfer after leaving Hamburg in June following the expiry of his contract.
The Holland international had a trial at Birmingham's Championship rivals Middlesbrough in the summer, but a move failed to materialise.
Casetelen is now hoping to catch the eye of the Birmingham coaching staff as he looks to win a deal at St Andrews.