The Serbian almost moved to the Riverside in the summer but the deal collapsed late on over medical issues.
The centre-back has since returned to full fitness and he played for Dortmund's under-23s last week.
And when asked if he could reignite his move to Middlesbrough in January, Subotic told Bild: "This is an option.
"Before then there is still a bit of time. My concentration at the moment is if I do not play games and get on the pitch, I cannot offer anything for Borussia Dortmund or any other club in the world.