I like Upamecano, but he is not at Koulibaly's level just yet.
People have to remember that they are playing against a very defensive team (not Barca and Messi) and he is playing alongside 2 other CB's. It is very easy to look half decent playing in a system with 3 CB'S, just look at David Luiz at Chelsea. Looked imperious in a back 3. Would like to see him tested in a back 2 and see what he is really made of defensively.
I have seen him a number of times and i do really like what i have seen though, however there is a very unganely style similar to N'Dombele, doesn't seem to have the best balance, but is pretty good on the ball.
He is definitely one for the future though, however Koulibaly is the one for me right now.