And its name is Andrew Bynum, and what it (or, rather he) does on the court from here on out will determine whether Brown's first season is a rousing success or an abject failure.
Because, for a Lakers team with Kobe Bryant at its lynchpin, anything less than a campaign that ends with another Larry O'Brien Trophy falls firmly in the latter category.
The Lakers' decision to hire Brown as the next head coach of the NBA's most glamorous franchise last year was met with plenty of second guessing by the sports media, both local and national.
How would the former Coach of the Year handle the pressure of succeeding the legendary Phil Jackson on the Lakers' bench? Would Browns' defensively-focused style of play mesh with the expectations of a team (and a fan base) so closely associated with the glitz and glamour of nearby Hollywood?
Most importantly, how would the new guy deal with Kobe Bryant?
After all, Brown was a significant part of the Cleveland Cavaliers regime that so thoroughly enabled LeBron James to become basketball's biggest brat.
And, thus, he was at least somewhat at fault for the franchise's downfall, rightly or wrongly.
As such, there was a fear that the Black Mamba would pull a King James on the supposedly weak-willed Brown, taking advantage of the new coach to have his run of the asylum as he sees fit.
Fortunately for the Lakers, Kobe's proven to be much more mature than LeBron in that regard, developing a strong-bond with a man who he's described as a kindred spirit of sorts while going out of his way to avoid criticizing Brown.
The Killer B's
Unfortunately, Bynum hasn't been so careful, haphazardly stepping right into LBJ's shoes under a big top that's settled its stakes around the Staples Center after previously engulfing Quicken Loans Arena.
Like LeBron with the Cavs before, Bynum is a darling within his own organization, the precious pick of upper management (i.e. Jim Buss) that's invested so much in him to make him a success. The team gone to great lengths to foster an environment in which the gifted big man would feel comfortable and be able to deliver on his considerable potential.
Which he has this season. Now that he's finally healthy, Bynum's managed to average 18.4 points, 12.2 rebounds and 2.0 blocks per game; earn his first trip to the All-Star Game, as the starting center for the Western Conference squad; and pull down 30 rebounds in a single game, as he did against the San Antonio Spurs on Wednesday night.
There's no question that Bynum has arrived, though his bad attitude has clearly accompanied him on the way up.
And with such rarefied air comes the opportunity for ego expansion, the leeway to test institutional limits.
Tickets for parking in handicapped spaces, ejections for needlessly arguing with officials, suspensions for slamming opposing players, launching perplexing three-point shots.
The acts of juvenility on Bynum's part aren't Brown's doing, nor did most of them even happy on his watch.
The fact remains, they are Brown's inheritances. Coaching the Lakers was never going to be easy for Brown, even if Bynum wasn't such a knucklehead.
Not Another Cav-tastrophe
Now that Brown's (and the Lakers') worst fears about Bynum have been realized, it's imperative that everyone work together to nip Bynum's petulance in the bud, lest they allow it to derail the team's pursuit of a third NBA title in four years.
Much of the responsibility rests on Brown's shoulders, if for no other reason than that he's the head coach, but he can't do it alone. He'll need Kobe and Pau Gasol to lend their assistance in keeping Bynum in line, especially now that Derek Fisher is no longer around to maintain order in the locker room.
Who's most important to the Lakers' postseason success?
It'll require a team effort, but if the Lakers can keep Bynum on his best behavior (or something close to it), they'll be as good a bet as any to win the West and crack the NBA Finals.
That much was made clear by Bynum's career-best outburst against the Spurs, and has been with each passing game during which he asserts himself as the best big man, if not the best player, on the court.
The question remains, will Brown let Bynum bully him like 'Bron 'Bron once did? Or will he wrest control of the team back into his own hands, where it belongs?
The answer will likely dictate when and where the Lakers' offseason begins.