With the news Jerome Boateng will be sidelined for most of the second half of this season, Bayern Munich's hopes of glory in the Bundesliga, DFB-Pokal and Champions League are in some danger.
The German giants have lost their best center-back and perhaps their best defender overall. And their options in his stead are scarce.
For Bayern to succeed, they'll need to adapt their approach to playing and winning games. In part, they'll need to borrow a page from the book of Louis van Gaal during his time at the club: To simply keep the ball and outgun opponents in attack.
It was a tactic that worked for Van Gaal in 2009-10, when the Bavarians reached the Champions League final in Madrid with a back five of Hans-Jorg Butt, Martin Demichelis, Daniel van Buyten, an inexperienced and out-of-position Holger Badstuber at left-back and Philipp Lahm on the right as their only defender of considerable class.
It was a bumpy road to the final, and the Bavarians conceded four goals to both Fiorentina and Manchester United in the knockout rounds, but they managed to equal their opponents in both cases and progress on away goals.
Bayern took a similar approach the following year and blew a 3-1 aggregate lead at home to lose to defending champions Inter Milan on away goals. As the saying goes, if you live by the sword, you die by the sword.
Bayern have a better squad now than in 2010 and 2011, however, and there's genuine reason to believe they can overcome their opponents with enough quality in attack. But for them to do so, they'll need their X-factor to be at his very best. That is Thomas Muller.
Looking around the Bayern attack, there is abundant class and everyone has his role. Douglas Costa is the primary playmaker, the creative spark on the wing. He demands the ball and can work magic with it, dribbling through defenses and creating scoring opportunities.
Arjen Robben and understudy Kingsley Coman have similar abilities but are perhaps not quite as adept in creating; the Dutchman is more of a scorer, while the teenager is a bit more balanced in both departments but for now has less responsibility in possession than Costa.
And then there's Robert Lewandowski, the classic No. 9; he's the focal point of the attack, the scorer of the most goals and hold-up man who challenges the center-backs.
That leaves Muller, the player whose role is so hard to define even many journalists struggle to truly encapsulate his brilliance. And many defenders just don't know what to do with him.
True to namesake Gerd, Thomas is the kind of player who can produce goals at any time and in any way. He is the X-factor for Bayern as they approach the business end of the campaign.
The best way to describe Muller is that he simply is effective. In an era in which the Bayern attack was defined by Robben and Franck Ribery ("Robbery!" read the headlines), Muller scored and assisted more goals than his more illustrious teammates.
He's been remarkably consistent, with Transfermarkt recording 141 goals and 113 assists in 331 games for Bayern in all competitions over the course of his career. This term, the figures stand at 21 goals and eight assists in 27 appearances in what is looking to be a career season for the 26-year-old. He just gets better and better.
Last time out, Muller didn't even have a particularly great game, yet he played a decisive role. He tracked down a hopeful long ball and won the penalty that put Bayern ahead against Hamburger SV. And later, he took an effort that probably would have been saved had Lewandowski not reacted to deflect the ball into the net.
These seem like flukes, yet they happen all the time. Consider how Muller appeared to badly mishit a cross from the right wing in September but had the fortune of the ball finding its way into the top-left corner of the net.
Defenses have long known exactly what Robben would do, and to his credit, the Dutchman has long been so good he's rarely been stopped.
With Muller, it's a different story; one never knows what he'll do, but one can count on him producing a goal. Sometimes it's a fluke, sometimes it's a moment of sheer genius one may not have thought him capable of. But one way or another, it happens.
Muller is also a player who isn't fazed by the big stage, and that could be critical toward Bayern's title hopes.
Whereas Costa and Coman have never been tested in the latter stages of the Champions League, the German has won the competition and the World Cup. In the latter, his 10 goals have him already drawing close to Miroslav Klose's tournament record of 16. And in the former, not only has he won, but he's appeared in three finals. He even scored the go-ahead goal in the 2012 final, a match he may have decided had he not been substituted.
With Boateng sidelined, Bayern have a tough task on hand from now until season's end. Yet they can still win everything. They have a team of world-class players and, among them, an X-factor in Muller.
If anyone is to produce the magic Bayern need to win a treble, he's their best bet.