Far be it for me to defend Benitez but I am sure this has nothing to do with him. For a fair few years player purchase, retention and letting go at Chelsea have apparently been driven by dark forces beyond the temporary incumbent of the manager's chair. Every comment I have seen by Benitez suggests this continues, whether it is buying new players or retaining or otherwise older ones. Read his interviews and he sounds as much in the dark as the rest of us.
I think his job description is "don't worry your pretty little head about buying and selling, or managing any aspect of the club. We'll give you the squad you need, you go and win stuff." Unfortunately, he's fast running out of trophies to win. Five and a half down (three not on his watch), just two and a half to go.