"The joy of playing is back again", said Mueller.
Bayern can open up a 14-point lead - at least temporarily - with victory in Leverkusen, but their fourth-placed hosts possess a number of particularly threatening players, with Heynckes highlighting Leon Bailey, Julian Brandt and Kai Havertz.
Barring a huge collapse in form - which doesn't seem likely with how Jupp Heynckes has been running the team since he took over the club earlier this season, which started that impressive run of form - Bayern are going to easily win the Bundesliga, and they'll break a lot of hearts along the way. "Neither of them are in the squad", said Heynckes, before including that German worldwide Wagner is the "primary alternative" to supplant Lewandowski in assault however a long way from sure to begin.
New Bayern signing Sandro Wagner, who made a late substitute appearance, set a club record with his league comeback for the Bavarians over 10 years after his previous Bundesliga game for them.
With Mats Hummels out with a groin injury, Niklas Suele partnered Jerome Boateng at centre-back.
Bayern were five points adrift in the league when Heynckes, 72, replaced Carlo Ancelotti at the start of October, but built up a commanding lead as key rivals RB Leipzig and Borussia Dortmund faltered.
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"He's very ambitious, very disciplined, " Heynckes said. "That is why everyone is working extra hard".
"It's tough to win at Leverkusen, but we played well here, although we need to move the ball more".
"Leverkusen is a young and very talented team which played very offensive".
However, Heynckes warns there is plenty of talent in Bayer's ranks, pointing to Germany winger Julian Brandt and teenager Kai Havertz, 18, who are both unsafe in front of goal.
He added: "Of course he and Lewy are different types of players but not a lot will change".
"I have said everything there is to say and won't say anything more", replied Heynckes when asked about his future in the midst of Bayern's hunt for a new head coach for 2018/19.