PSG announced on Thursday that the star forward would miss "up to three weeks" with a thigh injury, ruling him out of the February 14 clash in the French capital.
"I don't think he'll be out," Nagelsmann said, claiming the Ligue 1 champions had given a speculative diagnosis of the star's condition.
"I don't know what he's got, (but) I assume he's playing.
"What it says on the website is vague. If it's not a structural injury, I can't imagine him missing the game, but I don't know.
"I am preparing for the game as if he is going to play."
Mbappe, who has 25 goals for PSG this season including seven in the Champions League, was replaced in the 21st minute of his team's 3-1 win over Montpellier on Wednesday.
Alongside Mbappe, both Neymar and Sergio Ramos are currently under fitness clouds for the French side, but both should be fit in time for the clash with Bayern.
Nagelsmann's side have injury concerns of their own, with the long-term absences of goalkeeper Manuel Neuer, wing-back Noussair Mazraoui and defender Lucas Hernandez.
Forward Sadio Mane has been out since November but has returned to training, although he won't feature at the Parc des Princes.
"I am expecting him back in the middle or at the end of February," Nagelsmann said.
"It looks good and he has no more pain. But he'll miss the first leg against Paris."
With six wins from six matches Bayern are unbeaten in the Champions League this season, but have drawn their past three league matches ahead of Sunday's clash with Wolfsburg.
Bayern currently sit in first place in the Bundesliga, but are just one point clear of Union Berlin.
A further three teams -- RB Leipzig, Borussia Dortmund and Freiburg -- are within one win of the 31-time Bundesliga winners.
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