For me it comes down to the question is if Jack can outlast Zoro. I don't see that happening. Jack can fight long, and he has more techniques to his name but Zoro is a beast in terms of strength, and endurance. He will hold Jack and weather his attacks taking damage and dishing it back out. But Jack will get tired and Zoro will take him down in the long run. It is possible Jack may be able to get a lethal shot in but Zoro is just as quick as Jack is he's got faster than light reaction feats under his belt just like Jack. I may have initially thought to little of Jack but I just honestly don't see him beating Zoro in a drawn out fight. All of this is just because Zoro will be fighting Erza Scarlet in death battle and some people would've preferred Jack over her.