Lewandowski enjoyed his most prolific campaign yet last time out with 41 Bundesliga goals for Bayern to break Gerd Muller's long-standing single-season record.
His 48 in all competitions were the most scored by any player in Europe's top five league in 2020-21, followed by Kylian Mbappe (42) and Erling Haaland (41).
Poland striker Lewandowski has netted 294 goals in 329 appearances overall for Bayern since joining from domestic rivals Borussia Dortmund in 2014, including 203 in 219 league games.
Reports from Spain earlier this week suggested the 32-year-old is ready for a new challenge and Real Madrid is his preferred club should he leave the Allianz Arena.
Lewandowski is no stranger to transfer speculation, though, and Nagelsmann – named as Hansi Flick's successor in April – is not concerned by the gossip as the striker is under contract until 2023.
"The rumours about Robert have been around for ages, ever since he has been with FC Bayern," he told Sky Sport.
"I think it's somewhat normal when someone scores so many goals that almost every club puts out its feelers.
"On one hand Robert knows what he has in Munich, what he has in this team. Point two is his contractual situation – this is also clear.
"I've spoken to him. I'm not very fond on the first act with a player being to immediately discuss his contractual situation."
Lewandowski is currently enjoying an extended break after featuring in all three games for Poland at Euro 2020.
The former Dortmund striker scored three times as Poland suffered a disappointing group-stage exit after finishing below Sweden, Spain and Slovakia.
That return of one goal every 90 minutes was the fifth-best of any player to have scored more than once in the competition, behind Karim Benzema (87.25), Patrik Schick (81), Kasper Dolberg (75.33) and Cristiano Ronaldo (72).