I just can't imagine seeing the real Rocky in real life training for a fight against Tua and losing. You need legendary class skills like Ali's foot speed, Foremans power, or Dempseys ferocity. Tua's power gets slowly overpowered by Rockys hunger and toughness.
Charlie Goldman turned away Marciano the first time he was offered because Marciano was so poor in all aspects but punching power. You really don't see that version of Rocky losing to a 90 fight veteran amateur, bronze medalist, pro prospect who got snatched up quickly by the likely the most powerful trainer at the time?
Despite Marcianos shortcomings, he was still taken in by one of the best trainers in the business. What was that line in Pursuit of Happiness? "What would you say if I told you that I gave the job to someone who didn't have a shirt?" "..He must've had some really nice pants." Marciano's strengths must've really outshone his weaknesses. He had to have had some serious punching power, and Tua didn't look like he could take the kinds of punches at that point in his career.