I will disagree with you that Manchester United's performance was poor last season, if you said Manchester's United performance was poor at the beginning of last season I would agree with you. Last season was one of Manchester United's best seasons and United ended up winning a trophy. Chelsea is the only club that has had a poor performance at the beginning of last season throughout.
Obviously the last season, Manchester United wasn't that poor because they can still finish in 4th position and won Carabao Cup. But it was just a fluke, they have a same squad and same coach, but this season, they're perform really bad.
Chelsea had a terrible result in the last season, this season it might be Manchester United' turns.