Bayern Munich s Senegalese forward Sadio Mane. AFP
Speaking on Tuesday ahead of Wednesday's last 16 Champions League match in Munich, Nagelsmann said Mane was "still finding his rhythm" after "being away for quite a while."
"I don't think he will start but we still need to make a final decision on that."
Mane injured his leg in November and required surgery, keeping him out of Senegal's World Cup campaign. The former Liverpool striker has appeared twice off the bench in the league in the past fortnight.
"We are very happy to have Sadio back, he can bring a spark from the bench.
"Even if he doesn't start, Sadio is a player who can be -- and wants to be -- decisive from the bench."
Bayern come into the match with a 1-0 advantage thanks to a second-half goal from former PSG winger Kingsley Coman and only need a draw to progress to the quarter finals.
Twice Champions League winners in the past decade, Bayern have only failed to reach the quarter final stage on one occasion, the 2018-19 season where they were eliminated in the last 16 by eventual champions Liverpool.
Nagelsmann said attack was the best form of defence against "extraordinary" Paris striker Kylian Mbappe, who is set to start on Wednesday after coming off the bench in the first leg.
"It's key not to just focus on defence and sit back and think about how many excellent players PSG have.
"Instead, we want to send our excellent players onto the field, in their best positions, so that they can be dangerous and attack the goal."
Despite a difficult group which included Barcelona and Inter Milan, Bayern have won seven from seven in the Champions League this season, scoring 19 goals and conceding just two.
While Mbappe will return to the starting XI alongside World Cup winner Lionel Messi, midfielder Neymar will miss the match, and probably the rest of PSG's season, with an ankle injury.
Bayern veteran Thomas Mueller said PSG would "definitely be weakened" by the absence of Neymar and his "outrageous skills", but warned of his "favourite" player Mbappe.
"He doesn't just play beautifully, he's very effective. It's no secret the whole world likes to watch him play football.
"Tomorrow, we don't want to watch him we want to distract him and make him work. If it goes to plan, he won't have fun tomorrow.
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