There is obviously a coaching issue, even if their full defense hasn't played together much. The amount of space between their midfield and backline is ridiculous, it's like Frank wants the best of both worlds where he'll press with the attack and midfield but not leave space in behind. In reality that leaves his defenders isolated and in positions to make mistakes as soon as a midfielder loses the ball or the opposition breaks the press.
Seems like all the analysis here boils down to digs from both sides about spending or aging defenders or that it's 'all individual mistakes that will disappear when Mendy steps in'. There's pretty clear tactical faults in this Chelsea side that Lampard needs to recognize, regardless of whether his spending was prudent.