Liverpool performance were declining because Mane is leaving and several players were injured, at least if Liverpool players weren't injured, I think it's possible for them to achieve top 4 and qualify for champions league.
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Mané was already declining, just see how things are going at Bayern. But the rest of your comment stands.
Manchester City on other hands, I don't think Pep will let their players to leave the club, so I don't think the next season Manchester City will play bad. Remember, this is the beginning journey of Haaland in Manchester City.
Of course Pep will let the players go. Pep is known for ruthlessly axing players. Cancelo is almost certainly heading out the door, followed by Laporte, Silva and Gundogan and possibly even Kyle Walker.