Lopez is a rising star, he's gained lots of experience under Flick and turning into a future talent, he'll be a good addition to Chelsea but Barcelona is the problem holding this deal from happening, they too need their depth to be strong so I doubt if they'll give one of their finest youngsters to Chelsea especially now that the window is almost over and there's little time to scout a proper replacement.
Barcelona certainly does not want its mainstay player to leave but that does not mean it cannot happen, the only way for Fermin to leave is for him to decide for himself but from what I know he rejected Chelsea offer, he still wants to enjoy his good times with the Blaugrana besides what guarantee is there if Fermin leaves he can continue to be the starter for the English team considering that there are already many players who fill the midfielder position from that team, as we know Chelsea has a solid midfield, Enzo, Caicedo, Romeo Lavia, and Carney Chukwuemeka, this line is very competitive, and although it can encourage players to develop, it also creates uncertainty for Fermin career.