The thing about this fight is that it heavily depends on two factors: How likely is it for Retsu to go immediately for the K.O., and how likely is Lee to unleash the Gates or weight amps.
On a direct comparison of attributes, Rock Lee comes on top on most criteria, with Retsu's only saving grace being that Shaori can completely nullify strikes from people on his own level, so it should remain fairly effective in this scenario. Speed-wise is also interesting in that Retsu will slightly compensate the speed through higher skill, but that goes out the window the moment Lee takes out his weights or unleashes a Gate.
Retsu is more skilled, and measurably so by several parameters, but the thing is that skill can only take one so far, and the wider the power gap between people, hax notwithstanding, the less skill helps - and Lee himself is no slouch in this regard. As long as both remain in base, Retsu will hold a noticeable advantage, and a very dangerous one, as Retsu is one of the few Baki characters who do not really play around when fighting, even when he does not want to kill or really injure his opponents. In fact, he usually does go for pressure points or disabling amounts of force and technique from the get-go. The question is if it would be enough to take out Lee before he decides to unleash a Gate. Lee doesn't immediately go into a Gate the moment he has problems, and neither is he likely to just bust it out the moment Retsu deals one single good blow on him. The moment he does, however, Retsu can only win through a lucky blow.
I tend to think that this fight is inconclusive - I do not think any scenario is significantly more likely than the other, and the more I think about it, the more uncertain I am.