Arjen Robben has been the focal point of the Bayern Munich attack for the better part of the last two season. He's been a potent force that has carried many a result on his back for this team. After his injury in their 2-0 loss to Borussia Mönchengladbach, Bayern Munich will be without their Dutch talisman for several weeks. While several weeks is the club line, BILD recently reported that the injury loss may extend to six weeks wiping out most chances that Robben could play in the Champions League semi-finals.
Given the depth Pep Guardiola has in this squad, it's likely that Bayern Munich will be able to mitigate the loss of Arjen Robben through a combination of player rotation but at it's most basal level it will likely require the team to approach their upcoming matches with a different tactical plan in the coming weeks.
The Müller Swap
It's a good thing Arjen Robben has a ready made replacement for the right wing in Thomas Müller! Well not really. Moving Thomas Müller from the center where he's been a very integral piece of Guardiola's zone-based tactical plan to the wing isolates the German international and forces the offense to become more focused on direct play and his crosses. It also comes with caveat of being reliant on the health of Franck Ribery to mirror Arjen Robben's role on the left and to date that hasn't been Ribery's strongest suit.
While Ribery is capable of playing this role the Frenchmen is far better running direct at defenders and working in concert with his leftback and Mario Götze then cutting in from the wing, drawing defenders and being a pest. Part of that is his shot from range isn't as devious as Robben's and defenses can relax their pressure rather then having to commit.
Without Ribery and Robben though the teams predominant tactical vogue doesn't work. Aside from lacking two of their biggest offensive weapons, they simply don't have the sufficient number of players and they'd have to resort to something more akin to the 3-4-3 they spent much of the Hinrunde using.
Continuing the 4-3-3
In several recent matches, Pep Guardiola has shown a propensity to play with a 4-3-3 shifting away from the 3-4-3/4-3-3 hybrid he's played most of the Rückrunde. With the return of both Holger Badstuber to the backline and Philipp Lahm in midfield, this gave Bayern Munich a interesting conundrum in the center of midfield and a requirement that they address where David Alaba is going to play.
With Thiago Alcantara also in team training and close to seeing time on the pitch and Javi Martinez on the cusp of rejoining as well, Bayern Munich will have a collection of talent and depth in both defense and central midfield that they haven't enjoyed yet under Pep Guardiola. Juxtaposed to that is without Arjen Robben (and Xherdan Shaqiri on loan), Bayern have solely 4 healthy first team attacking midfielders (I'm not counting Claudio Pizarro here). With Franck Ribery's continued issues with health, there's not much incentive for Bayern Munich to play all their attacking talent at once as they've been want to do in the past.
That's where the glut of defensive midfield players comes in as Bayern Munich can now deploy a midfield of Schweinstegier, Lahm, and Alonso adding in Sebastian Rode and Thiago for depth and rotation. Add David Alaba to the mix when Juan Bernat plays and Bayern Munich have the midfield resources (and quality secondary runs from Schweinsteiger, Rode, and Lahm into the box) to sustain their level of play without Robben (and potentially Ribery).
Couple that with the fact that Mario Götze has struggled this season whenever he's been deployed on the wing having David Alaba allows Guardiola to play the advanced leftside wingback role he prefers (rather then a normal attacking fullback) and gives Pep the ability to drop Mario Götze into the middle as Lewandowski drifts left in combination with Müller's attacks on the right and his ability to move in and play off Lewandowski in the middle.
Continuing with this style of 4-3-3 might be the best option at both retaining a strong rotation policy and playing to the squad strengths Bayern Munich have right now.