I think it would look a lot like Tyson vs Pinklon Thomas, with Mercer shipping a lot of punishment before a mid to late round stoppage. Mercer is tough as nails, but, he is simply too easy to hit. That would ultimately spell disaster for Ray. Tyson was not fragile like Morrison or Damiani in the event that Ray landed some good shots.
After Tyson left Rooney after the Spinks fight his technique degraded. He wasn’t quite the same fighter though with maturing at some future point he may have hit a peak beyond the Spinks fight not sure. His defense for one was not the same he lost a lot of his head movement. That could be dangerous vs Mercer who was powerful tough as nails and never gave up. Slight advantage to Tyson but a dangerous fight for him.