Upamecano was one of the standout players in both the Bundesliga and Champions League last term, as he helped Leipzig finish third in the German top flight and reach the semi-finals of Europe's elite competition.
The 21-year-old was rewarded for his performances with a bumper new contract in July, which saw him commit his future to the club through to 2023.
However, the centre-back has now emerged on the radar of a number of high-profile potential suitors, including Premier League giants United and Arsenal.
The Red Devils are reportedly considering a summer swoop for Upamecano, who refused to rule out a possible move away from Red Bull Arena when quizzed on his future before the international break.
"I am targeted by several clubs, I speak with clubs, but I have time. We will see what happens in the future," he said.
Nagelsmann insists that the France international will continue to form a crucial part of his plans throughout the 2020-21 season, but concedes that the influential figure could move onto a new challenge next summer if he maintains his current trajectory.
"He [Upamecano] will be here for the season. I'm sure about that," the Leipzig manager told ESPN. "As for the future, I'm not sure as he is very talented.
"Lots of clubs will want to buy - next season it could be real that he leaves the club. But this season he will stay."
Nagelsmann expects the biggest clubs on the continent to continue targeting his best players, but he is also confident that Leipzig will be prepared to dip into the market for new talent to ensure that the squad keeps progressing in the coming years.
"It is normal when you work with young guys that clubs will try and find young talented guys in the transfer market," he added. "It is normal that bigger clubs will try and buy these players.
"You have to deal with that, you've got to be one step forward when you buy new players or find new players.
"Our scouting should be good and we have made a lot of good decisions in the transfer market."