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I think it is reasonable that Mauricio Pochettino has not been able to fulfill his promise because Chelsea did experience a serious decline in the previous season and the coach also needs time to be able to fulfill what is expected and since the time he took office as Chelsea coach he has improved Chelsea's performance and managed to finish better than the previous season, even though his contract at that time lasted 2 years if Todd Boehly had been more patient, Chelsea could have been in the top 3 in his second season, unfortunately the club was not that patient and they fired Pochettino at the end of the season.
If you say Chelsea faced serious criticism in their past season, then what are you going to say about their performance in the previous season before Pochettino comes in as a manager? The thing is that Chelsea wants a quick result, and it will be hard to achieve because I don't see why a coach is signing a team, and you will be expecting a trophy from that coach the same season he got in. No man is impossible; at least there should be enough time to see if he can be able to do it in two seasons.
Furthermore, Pochettino was able to restore Chelsea's performance towards the end of last season. I don't know if you see that, so it is surprising that you fired him after he failed to bring the trophy home. I think he tried his best for the little time he spent, and he deserves more time.