This defeat is big and unacceptable because Manchester United and Liverpool are traditional competitors in Premier League and none of them want to lose a match to their opponent. Manchester United can not use any word to heal their pain from this defeat which is a big shame too.
The defeat against Liverpool was a really unacceptable thing for Manchester United. This is because they played so badly in this game.
Look at Manchester United's passing network compared to Liverpool. They looked so bad. There was no creativity from the midfield. Too many back passes. Rashford got passed only once. The full-backs were too high on the pitch. And Liverpool's defenders isolated their forwards. With all the problems above, they deserved it.
Dude, focus on Manchester United's gameplay, not the rivalry between them and Liverpool.