We didn't find out who attacked Roman Reigns tonight on SmackDown, but it sounds like all will be revealed on next week's show.
After seeing Daniel Bryan and Rowan coerce an admission out of Buddy Murphy that he lied about seeing the latter in the forklift, The Big Dog confronted the former Tag-Team Champions just as the episode was coming to an end. Bryan demanded an apology, but when Roman seemed ready to throw down, he told him that they'd been conducting their own investigation and will bring him the culprit next week.
We already know Bryan and Reigns are scheduled to face each other in upcoming house show matches, so there's still a good chance Murphy was telling the truth. We'll just have to tune in next week to find out.