The 27-year-old former Serbia international central defender agreed to join the Teessiders from German Bundesliga outfit Borussia Dortmund in a 10million euros deal during the transfer window.
Subotic had suffered with thrombosis in his arm during the course of last season, but that was not thought to be an issue.
However, when he underwent a medical at Boro a rib problem was detected and the move was immediately called off, with Subotic returning to Dortmund.
He had surgery on the injury and has successfully returned to action, with an outing for Dortmund's Under-23s negotiated without any setbacks last week.
Subotic is now in a position where he can think about his future and has not dismissed the possibility of resurrecting his move to the Riverside Stadium.
He 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.
"That's why this is the most important thing now."
It remains to be seen if Boro boss Aitor Karanka will want Subotic given that he brought in Calum Chambers on a season-long loan from Arsenal, and he has formed a successful partnership with Ben Gibson.
Dani Ayala and summer signing Bernardo Espinosa, who has yet to make a Premier League appearance for the club, are also waiting in the wings.