Upamecano was at fault for PSG’s opener after gifting possession to Moise Kean, who teed up Angel Di Maria for the visitors’ opener.
The highly-rated 22-year-old France defender also committed a penalty, which Di Maria missed. Luckily for Leipzig, they recovered to run out 2-1 winners over Thomas Tuchel’s men.
Upamecano was also part of the Leipzig backline that conceded five goals against Manchester United last week.
Nagelsmann has noticed errors creeping into Upamecano’s game and has asked the reported Liverpool and Manchester United target to remedy the situation.
"The mistakes he's making are all starting the same way," Nagelsmann said. "He wants to pass the ball to the attackers right away once he's won it. That simply does not work all the time.
"I can see a learning curve. Against Hertha, we've had similar situations, the same at the start of the second half against Gladbach, where we end up defending one against two players.
"He has to recognise when he can play the ball forward and when not. He is used to playing the ball forward fast to create a counter-attack.
"He grew up like this during his first year at Leipzig, when this was the main tactic. Now the next step has to come."
Nagelsmann added: "In general, Upamecano is on his way to world class, but he has to stop these things, obviously. He knows it himself, he is very critical of himself.
"It's a learning curve, he only has good intentions and wants to create attacks for us. He simply needs to recognise the right moment to switch from defense to attack.
"Today he should have played the ball to Peter Gulacsi.
"It's the same when attacking. There are moments where you simply have to get the ball back, and then there are moments that are about scoring a goal.
"Recognising this situation is a learning curve which he will succeed in, simply because he is that talented. I hope he will succeed soon, because then we have less distance to go."
