Chelsea surely have potential problems unless they buy
Goalkeepers: Courtois, Begovic, Blackman
Defence: Ivanovic, Azpilicueta, Terry, Cahill
Midfield:Ramiers, Matc, Mikel, Fabregas, Hazard, Willian, Oscar Moses, Cuarardo, Salah
Forwards: Costa, Remy, Falcao
U21: Zouma, Chalobah, Ake
Admittedly they are well stocked in midfield but surely weak in defence and up top, but as it stands they are a couple of injuries away from having an issue. Costa is already struggling and Falcao and Terry has to be coming to the end.
Chelsea's defence only looks "weak" because you forgot to include Zouma (he's much more than simply an U21) and Rahman, their defence is also fantastic at clocking up game time without looking bothered, it's well conditioned and seems to avoid injury, and it's anything but weak. Chelsea's squad is by far the most balanced, and simply the best in the league, they're the team to beat, the only real weakness they have is that they rely on their special players a little much to provide the spark that their conservative system starves off, an injury to Hazard and less so Costa could cause them to be a little impotent, turning wins into draws, but that's about it.