Nagelsmann, whose sacking in March 2023 led to Tuchel's arrival, was tipped to return to Bayern, but has now extended his contract with Germany until 2026
"The most important thing will be to win the challenges otherwise tactics will mean nothing," he told a news conference.
PSG have been eliminated in the quarter-finals twice, in 2013 and 2015, by Barca, who will start the tie as favourites.
"We know this team very well and that's an advantage. This tie is the perfect occasion for the club to grow," Emery said.
He added that he would make a decision later on Monday on whether Argentine midfielder Javier Pastore, who has resumed training after an injury, would be called up in the squad.