I'm pretty sure you already know the reason
but for those who don't here why:
1- there are not one, not two, not three, but eight new players in the line up. It will take them a while to adapt to the football style. WHile I think guys like Gerrard and Huntelaar have the mentality and the ability to adjust their play style right away to suit us, for others it would take time( 1 month, 2 or a whole season) and thus it could hurt our chances of being a good unit.
2- Eight new players in the line-up. Even if we were to assume that they would adapt right away to the Italian style, with the exception of the Juve players, noone of those players have played together. It would take them atleast a few months to gel together and for DD to find the best formation and tactics for that team. Those few months in which the team is gelling together we will not get much good results and it could decide our chances of winning the title or even a CL spot.