3. Lacking an attacking midfielder who can score goals:
Bayern have desperately needed an attacking midfielder capable of scoring goals in the last few years. Muller has scored 181 goals in 458 matches for Bayern in 11 seasons but needs to be more prolific now.
When Robben and Ribery cut inside from the flanks, they usually look for a player on the edge of the penalty box to play the ball to. Bayern have lacked a player who can surge into the opposition penalty box and take shots into the goal after having received passes from Robben and Ribery.
James Rodriguez was brought in a loan with this very intention but has impressed in patches only so far. He has scored 11 goals from 50 matches for Bayern and would definitely be disappointed with his return.
Therefore, if Bayern play in a 4-4-1-1 with Martinez and Renato/Thiago as the two central midfielders, Muller would have to play in the hole behind Lewandowski and take those shots into the opposition goal from the edge of the box. Rodriguez can be used as a substitute for Muller.