I believe that James Buster Douglas pre Tokyo, wins this fight very easily, Buster Douglas is too young to lose to Cooney who looks good beating up on old fighters to make himself look good. Cooney has terrible balance, he might trip over his feet, he does not look menacing to beat a young pro who might laugh at him every time he misses. Douglas takes him out late, TKO 11. I have never been impressed with Gerry, except when they dug up an old corpse to get into the ring with him. His management did not match him with top contenders like a Greg Page, Tim Witherspoon and a Pinklon Thomas or they would not have made any money off of his carcass. As I have said, Jerry Quarry was a much better contender than Gerry Cooney, he fought the top guys near his age, not broken down fighters who have seen better days like Ken Norton and Jimmy Young like Cooney did.
The OP specified "undefeated" Gerry Cooney, which means the bout would have to take place before June 1982. At that point Douglas had eleven fights under his belt, including a two round TKO loss to debuting 271 pound David Bey. Buster was a guy who took a few years to turn into a quality fighter. In 1981 or the first half of 1982, Cooney takes it easily via an early knockout.