Pep Guardiola is in his first earnest title race at Bayern Munich

Saturday's "Klassiker" between Borussia Dortmund and Bayern Munich doubles as the pivotal moment in this season's Bundesliga title race. The two protagonists in that storyline come as no surprise. The strange part? It's March.

Last season, Bayern held an 11-point lead on Wolfsburg ahead of Matchday 25. Two years ago, the gap was 20 on Dortmund. In both campaigns, the Bundesliga was done and dusted well before Easter, but this year could be a different story.

Most pundits had already penciled Bayern in to become the league's first four-peat champion, but Wednesday's stunning loss to Mainz --€“ Bayern's first at home this season --€“ coupled with Dortmund's win at Darmstadt suddenly leaves the Black & Yellows a chance to cut the gap to just two points at the raucous Signal Iduna Park (12 p.m. ET, live on FX, FX NOW and FOX Sports GO).

"We know it, and Bayern know it as well," said BVB left back Erik Durm. "If we win on Saturday, it'll get really tight." 

Bayern head coach Pep Guardiola

This is unchartered territory for Guardiola. Never before has he faced such a challenge in the Bundesliga this deep into the season. Everything we've come to expect from Guardiola's Bayern in the league is suddenly under threat. So, how did we get here?

One must give credit where it's due. Dortmund have played lights out in 2016. With nine victories and no losses in ten games across all competitions since the turn of the calendar, Thomas Tuchel's side is heading into the German 'Clasico' confident, hungry, and with the momentum clearly on their side.

In contrast, Bayern's gotten off to a somewhat slow start in the new year, partly due to less efficiency in front of goal, but primarily because of a slew of injuries to the back line.

Bayern Munich's Jerome Boateng is treated by team doctor Volker Braun

Without Jerome Boateng, Javi Martinez and Holger Badstuber, and until last week, Mehdi Benatia, Guardiola has been forced to utilize David Alaba and the green but talented midfielder Joshua Kimmich as his top two center backs. That move has worked out fine against the likes of Augsburg and Darmstadt, but less so at Juventus once the desperate Italians pushed forward in the second half.

The second-half substitution of Benatia, of all people, destabilized Bayern in that game and invited Juve's resurgence. The Moroccan international was not much better against Mainz, and so Guardiola will likely go back to Alaba and Kimmich, a proposition that should have Pierre-Emerick Aubameyang and Marco Reus licking their chops.

Dortmund know they must win to truly make things interesting, and they'll throw everything they have at that makeshift backline. The question now is, how will Guardiola and his players respond?

Borussia Dortmund manager Thomas Tuchel

Guardiola's teams, even at Barcelona, have always looked a bit vulnerable against strong attacking sides, and once Bayern have fallen behind in big games, they've quickly unraveled. One must only look to Bayern's last two Champions League semifinal ties against Barca and Real Madrid for proof.

To counter, Bayern will look to do what they always do, and what they did so brilliantly against Juventus for 60 minutes: keep the ball. In that regard, however, Boateng's absence could prove critical two-fold. It was Boateng's steady supply of long balls over the top that turned out to be Guardiola's trump card over Tuchel in their first meeting this season, a 5-1 drubbing at the Allianz Arena.

Without his defensive rock and deep-lying playmaker, Guardiola now faces his sternest test in three seasons in Germany.

Should he lose, the Spaniard's final three months at the club could make for an uncomfortable time, indeed. The whole of Germany, meanwhile, would finally be able to marvel at an actual title race again, not just for the rest of March, but perhaps April and May as well.