It is still only the first step back into the team. Sometimes after injuries some of the players need their time to get back into their best possible condition. Now I don't think Harry Kane should take too long and he is badly needed since Bayern doesn't have a great second striker. Muller is by no means on his level and his comeback is urgently needed. I don't know whether they would have won against Mainz if he had been back, but the would have been better off.
Kane is very important with Bayern Munich and he replaced Lewandowski in the club very successfully. He left Tottenham in order to chase for titles, and failed in his first season for Bayern Munich with very surprising amazing season from Leverkusen.
In this season, Leverkusen decrease in their performances and no longer play excellently like the last season. It gives opportunity to win Bundesliga back to Bayern Munich and Harry Kane. Now they have 4 points more than Bayer Leverkusen that eventually climbed back to a second position above Frankfurt, Leipzig, Freiburg, and Stuttgart.
But what I noticed now is that as soon as Kane is out, Bayern doesn't have a strong backup. I don't know if the way how their squad is constituted is really sufficient for a team that aims for the Champions League trophy. The Bundesliga is not a problem, but playing the best teams with Muller as a striker or center forward?
They have some players who can take over, but not like Harry Kane does as he is all about the center forward position and otherwise they have a change in their system, which I think a team shouldn't want to have too often.
Bayern Munich have to figure out better replacements, alternatives for matches without Kane, but replacing a key player is never easy for all clubs and all coaches. This will be very challenging task for Bayern Munich and their coach Vicent Kompany but it's right that this task will decide their success in this season, in Bundesliga and Champions League as Harry Kane won't be able to play all matches in this season for them.