Germany, who end their tournament preparations for the Euros on home soil against the Greeks, must reduce their squad to 26 players by midnight on Friday to meet European soccer body UEFA's tournament deadline.
Nagelsmann has also included four keepers in his preliminary squad.
"We have our starting 11 in our heads but the performance in the match and in training must fit," Nagelsmann told a press conference on Thursday.
"Overall the squad decision has been taken but I won't announce it, nor talk with those affected because in the worst case someone is injured tomorrow and the player affected needs to slip back in and it would be silly to have that chat now."
"The decision has basically been made, we'll announce it after the game tomorrow," he said.
The Germans, eyeing their first major international title in a decade after a disappointing run since winning the 2014 World Cup, kick off Euro 2024 on June 14 with their opening match against Scotland in Group A.
They also play Hungary and Switzerland in their group.
"We are on a good path. We are reaching 100%," Nagelsmann said, adding that forward Kai Havertz was their likely starting striker over Borussia Dortmund's Niclas Fullkrug.
"We are happy that Fulle (Fullkrug) is back. It's good that we have another striker in the squad," Nagelsmann said.
Fullkrug joined the team this week after Dortmund's Champions League final defeat to Real Madrid on Saturday.
"The roles are clear. If Kai performs, he will have the edge. He has to perform. Fulle will get his playing time and will be able to score goals and cause a furore."
"Nothing is set in stone. You have to perform to get it set in stone," said Nagelsmann.
He also had good news on winger Leroy Sane, who had been nursing a bone injury for weeks.
"He is an option for tomorrow. He has done two training sessions. However we cannot calculate him in for 90 minutes of every match. He has found a good way when the tension is there (in his injury) to take it out."
"It could be that he can't play all matches through but he is about 100%."
