The three European heavyweights were drawn on Thursday in Group C along with Czech club Viktoria Plzen only weeks after Lewandowski, Bayern's second all-time top scorer, joined the La Liga club.
"I don't know if I grinned when it (draw) happened but it was a bit of Murphy's Law," Nagelsmann, referencing the popular adage that says anything that can go wrong will go wrong, told a news conference.
"To be honest I sort of expected it to happen. It is certainly the most challenging group. In my head I have already directed the film of how it should go and I am confident we will push through," he said.
Poland forward Lewandowski enjoyed a glittering eight years at Bayern, winning almost every major domestic and international club trophy, including eight Bundesliga titles with them as well as the Champions League and the Club World Cup.
The striker scored 344 goals for Bayern in 375 games, second only to club great forward Gerd Mueller. He joined Barca in a deal worth €45 million euros last month after forcing a move from the German club.
His departure angered some Bayern fans but Nagelsmann said the player deserved the highest respect on his return to Munich with Barcelona in the Champions League.
"I am not interested in what has been said or written," the coach said. "I think there will be applause. I don't like whistles against anyone. I want that fans receive him in exactly the same way they celebrated his goals here. He absolutely deserves that."
"It is a professional sport and it happens that you leave a club. It's not that he left after a season at the age of 21. He achieved a great number of things here," Nagelsmann said.
