So, you have conclusive evidence of his footballing brain being low? How do you measure it? Is it like an IQ-test, or is it based on pure guesswork?
His footballing brain from a defensive position seems to be just fine as he is our best defender. From a ball-playing perspective it also seems fine as he has realized his limitations - one would be wise to do similarly. Thus, I would say his abilities as a ball-playing centre back are less than average for a top six team. On the other hand, he makes very few mistakes with the ball and usually, his pass percentage is quite high.
Playing out from the back, would you want us to use Sam Johnstone from our academy over DDG, as DDG's ability to play out from the back is also quite limited.
All that being said, I really do like what I'm seeing from Tuanzebe. He is still very young, but looks calm and composed. He is very fast, but his aerial ability seems less than desirable. If you were to have your way and we sell Smalling (who of course would go straight down to playing league 2 football, at best), we would need someone great in the air as all of our centre back options, bar Smalling, simply are not good enough in the air as a pairing.