We spoke with Rockhold, who says his shin was surgically repaired after his fight with Yoel Romero back in February ... but the wound reopened while he was training this past weekend.

So, Luke went back to the doctor who needed to sew his shin shut -- and the whole thing was insanely graphic ... we're talking lots of blood and gore.

There's bad news and good news ...

The bad news ... Luke says he can't fight again until at least October, so the fight he was targeting with Alexander Gustafsson is off for now.