Imho, I agree with the guys saying loan first.
They will be 18 in a new country, new league, new team, etc...they need to ease into it first and a year at strasbourg will do them good before joining the squad at Chelsea.
They probably can compete now if given enough game time but that's not possible at Chels. Like Carney...for his next step in development he'll need to play 30-35 games a season according to Maresca. How much game time will these 18yr olds get in our squad. A year at strasbourg won't only help them settle in but will also help their development, confidence, game time and gain as much experience as possible before becoming understudies to the squad the following year. Easing them into PL football is an important part of their development.