#1 Managerial Policy
One of the most underrated factors in a club's success is the synergy between the club hierarchy and the manager. It is important that both are at the same page not just in the direction the club needs to take, but also the ways of going about it.
For years Chelsea have relied on a 'hire and fire' policy while bringing managers with pragmatic and result-oriented working styles. In Maurizio Sarri the club has gone with a manager who has a particular philosophy and prefers to play within his own system. Such managers are the ones enjoying the most success in recent years. However the cost is that the results aren't immediate.
Chelsea need to either fully embrace their own departure from tradition or steadfastly stick to it. Being on the fence about this is about the worst thing the club can do. Considering he did not even have a proper pre-season to work with his players, firing Sarri at the end of just one season would benefit nobody.