I don't know, i see Chelsea as tired, perhaps the Club World Cup has drained all their energy, rightly as you say the only thing is to see the start of the championship and see if they can recover and give their best as they did last season.
In the meantime, let's wait and see.
Tiredness does not really seem to be the case with Chelsea it is just them facing a totally different challenge from what they were facing before the season started so they need to probably comeback into the premier league reality which is as usual a very competitive one with teams of very good and threatening depth, a case where even underdogs can turn out a threat to yo if you ever take them fir granted, Chelsea will surely get back in form full soonest and possibly put up better performance than what they have now.