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The fact that Chelsea are is letting go of some of their best players to other clubs both in Europe and in Saudi Arabia should send a strong message that the new manager at the club Mauricio Pochettino has marked these same players as players that are surplus to his requirement.
The transfer season isn't yet finished and I'm very sure after mass clearance of players, Chelsea will definitely sign their replacement which will be the kind of players that the new manager will recommend to suit his style of football
The massive overhaul Chelsea has made in the last 2 seasons is very risky. They have spent a lot of money to recruit new players but their income is not balanced. If you visit the site transfermarkt then you know their finances are out of balance.
Mauricio Pochettino shouldn't be rushing to purge the old Chelsea squad and replace it with a new one, but I think he may have the boss' approval already. Mauricio Pochettino seems to need a lot of time for his new team to really be able to put in good form, so I don't think Chelsea will be as good as expected next season.