If Parker stayed focus for every second of the fight and deployed the correct, disciplined, safety first strategy, he could jab and move his way to a decision win. Even then Tua would have a solid punchers chance. If Parker lost discipline and engaged David, Tua becomes the clear favourite.
As long as Paint Dry Parker sticks to the jab vs the One Trick Pony who follows you around like a puppy, he has a good chance of taking a UD. Tua had to come from behind to to get to Maskaev/Izon/Oquendo and Rahman, the first three ran out of steam late and the latter is glass jawed, Parker has a decent chin and usually does not run out of gas late. However, Tua version Ike would pulverize Parker, so would Tyson, Bowe and Lewis. Tua on the other hand sends Dillon Shyte into the wonderland.
Tua has better chin and punch, Parker has better footwork (Tua doesn't), so basically I think like this:
Tua-Oquendo would be the closest comparison to the way I see Tua-Parker unfolding. Tua come from behind stoppage somewhere around round 9 or 10.
I've seen Tua outboxed by fighters that were nowhere near the skill set of Parker, Parker by a very wide UD. Tua's only chance would be by KO, and Parker isn't gonna get one shot ko'd. You'd have to put a sustained beating on him like Joyce, and I have never seen Tua put a sustained beating on anyone in his entire career, he just clips guys.