I think Xavi also selected Alonso and Christensen because they at least have some chemistry playing together in Chelsea although in a different system.
Defense is still a work in progress with a lot of new pieces trying to phase out two long-term members of defense which most of us wanted for years. They can't just click over the night so Xavi will need a few months to figure out combination(s) that work together the best and then build their understanding. And I'll be patient and give him and players time as I'm sure we'll be better longterm.
Probably won't be enough to go far in CL this season (don't think most of us expected that anyway) but should be more than enough to win La Liga which is my main goal.
Also, conceding two against Bayern (basically from their only 2 good chances as well) is not the end of the world, over the last few seasons we've conceded 2 goals (and could have more in many games) against worse teams.
As for Alonso I don't think he was as bad as people make him out to be (would actually say he was pretty good in the first half with the rest of the team outside of forwards) especially for his first start against tough opponent. Would start him every day of the week over cry baby.