Muller, who is a rare one-club player after having come through the Bayern youth system, has been left on the bench in recent weeks under Tuchel and has played only minor substitute roles.
He did not start either of their two matches against Manchester City as they were knocked out of the Champions League in the last eight and his lack of game-time has fanned speculation he could be looking for a move away from Munich at the end of the season.
"Thomas is the one who will mainly decide that because Thomas is Bayern Munich and Bayern Munich is Thomas," Tuchel told a press conference.
"He is playing for his 12th German title. That is what we are all focused on. I consider everything else an exaggeration and missing the target."
"He remains a key part even if he came from the bench a few times recently. From that to go and talk about a career end is widely off the mark."
Tuchel said the 2014 World Cup winner has had a reduced programme in recent days.
"He has some knocks at the end of the season which is normal," Tuchel said.
Muller is a crowd favourite in Munich and the last Bayern regular to have transitioned successfully from their youth system.
Bayern, who are in top spot in the Bundesliga, a point ahead of Borussia Dortmund with three games remaining, host struggling Schalke 04 on Saturday.
"Thomas is focused and wants nothing else other than winning the title with us," Tuchel said. "He is ready for tomorrow."
