Even if he had no defense, he still went the distance with Holmes (which Cooney actually failed to do) and was no match for Tyson. Cooney also had no defense to speak of.
Mike Tyson was an angry, hard attacking, wrecking machine in his prime. I just cannot see Cooney holding him off for very long at all, because he was just not skilled enough to do that against someone who jumps on him with fast hands and power the way Mike would.
Mike takes this, in 3 rounds or less. Gerry might get his attention with a couple of hooks if they land. But I doubt it happens, and Mike gets him out of there for the full 10 count.