I can't say that last season the Red Devils had a much worse result than this season... They got a place in the Europa League and didn't do too badly, but they lost to the reigning champions, Sevilla... Chelsea have had a really bad result this season, and if they qualify for the Champions League next season, it can probably be called a good result... Man U, on the other hand, are playing at an average level...
comparing the two Man united teams with Chelsea it is clear that last season Chelsea experienced a very bad decline in performance when compared to the previous season but Man United had a normal performance that did not have a decrease or increase last season.
I don't know what caused Chelsea to experience a decline in performance since the old coach was sacked and replaced with a new Chelsea player's performance did not experience any improvement.
it will be Lampard's responsibility to improve the performance of Chelsea players even though there are rumors that the Chelsea coach will be replaced again by Mauricio Pochettino.