Mourinho was spotted at the Olympiastadion as Dortmund took on Hertha in the Bundesliga, leading to questions over the reason behind the Portuguese's visit.
Tuchel saw no reason for concern, though, and explained Mourinho was invited by chief executive officer Hans-Joachim Watzke to watch Dortmund in action.
"No, I did not know Mourinho attended the game. This is not okay. Watzke is sitting next to another coach...," Tuchel said.
"No, I am just joking. Of course, I knew he would be there. It's not a problem for me.
"I would not be okay with it if he would replace me as Dortmund coach next week, though!"
Tuchel took charge of Dortmund ahead of the 2015-16 campaign and has guided them to 14 wins from 20 Bundesliga games as they sit second in the table, eight points behind leaders Bayern Munich.
Dortmund meet Stuttgart on Tuesday in the DFB-Pokal, while Porto await in the Europa League on February 18.