"The coach has taken the decision and it is absolutely correct," Dortmund's CEO Hans-Joachim Watzke told Sky.
The 27-year-old Aubameyang, who has scored 10 goals already this season, is suspended and was left out of the squad against Sporting.
Colombia striker Adrian Ramos took the Gabon captain's place up front in a 4-1-4-1 formation with a win to put Dortmund in the last 16.
"We completely accept there will be speculation, but internal means internal," added Tuchel in refusing to give more details.
Aubameyang has a Dortmund contract until June 2020, but the suspension will alert Europe's top clubs, especially Real Madrid.
The Gabon striker has made no secret of his desire to play for the Spanish giants having promised his now deceased grandfather he would one day play for Real.