Even though I also think substitutions were one of the causes of Liverpool's defeat, I can't blame Klopp at all. Because we know that M. Salah is actually still quite susceptible to re-injury. Even in interviews last month or a few weeks ago, we knew that Klopp would try to play their core players more carefully, especially Salah. And maybe Liverpool attach more importance to the Premier League. Although I know Klopp also has ambitions in the FA Cup. But it seems that Klopp also realizes that with so many injured players, Liverpool must save more energy for the Europa League and Premier League matches. But I don't know what is clear, I am also quite disappointed with the results, although I can understand that. Oh yes, Man United seems to be starting to have quite high self-confidence again.
Their fans expected too much as with many injured players, it's hard for Klopp to manage the team with good results. They had an impressive match against Manchester City but only had a draw in Premier League. They had this match against Manchester United, which is not actually bad, but they failed at the end. Generally Liverpool played well and they failed because they lacked of luck in this match plus a serious fight from Manchester United.
The bad news for Liverpool is not this single defeat but they get three attacking players injured from this match, Diaz, Nunez and Gakpo. This will harm their squad depth in coming matches and their fans must hope that things will go well with the team in Premier League and Europa League.