I think the idea from Tuchel is that City's defenders will be occupied with keeping Lukaku under wraps. Meanwhile, Ziyech and Pulisic can attack the space between the CBs and FBs. Chelsea's fullbacks can then overload
However that's contingent on a lot of things going right for Chelsea. First, will they be able to keep the ball well enough to hurt City in transition?
The idea is to go long because of the City press but Lukaku is showing he isn't capable of decent hold up play. Good for nothing oversized joker incapable of doing the basics.