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
Barca's in-form 16-year-old Ansu Fati became the club's youngest player to feature in a UEFA competition when he started but failed to make any real impact before being replaced by fit-again Messi just before the hour mark.
Even after the Argentine took the field it was the Germans who were more dangerous, with captain Marco Reus' 57th minute penalty saved by Marc Andre ter Stegen and Julian Brandt rattling the crossbar with a thundering shot in the 77th.
In the group's other game Inter Milan were held to a 1-1 home draw by Slavia Prague.