The 29-year-old, who is currently a free agent, had a trial with the Championship outfit last week before returning to the Netherlands.
"He's gone back to Holland but I think he'll be coming back for a week or so," said Mowbray. "He's a very experienced, talented player who hasn't played much football recently. It's just whether we give him that platform to get fit because he has undoubted talent.
"He went for a lot of money to Hamburg from Feyenoord, he's played at a real good level and I can see that in flashes and yet if you're not fit it's very difficult to reproduce that.
"So there's a fair chance he might be coming back to train with us for a week or two and see if we can get him up to speed."
Castelen has been capped 10 times at senior international level, scoring one goal.