The absence of Nunez will definitely be felt, I think Firminho will have to come in to replace Nunez. Liverpool have a stronger attack that can penetrate United's shaky defense. Harry Maguire and Martinez will have to work so much and be on their heals through out the 90mins of the game. I hope Manchester United can get a point from the game to boost their confidence for the next round of the competition.
Yes, Firminho will replace Nunez. I think there will be no problem with Liverpool attacking. I even think that Firminho is still better than Nunez since he already played with Liverpool for a longer time. Firminho can play more solidly as a Liverpool attacker, he knows Liverpool play of style better because he already played with the same teammates in many matches. Because of this reason, I think Man United has no chance to get a point. I predict they will lose again in the upcoming match.
Roberto Firmino has been playing at Liverpool since 2015, it's true as you said that Firmino knows the Liverpool game better than Darwin Nunez.
After all, Firmino is 30 years old, which means he is more experienced in controlling emotions when playing. Unlike Nunez, whose emotions are
easily provoked by opposing players when frustrated that he cannot score a goal. I'm also very optimistic that Liverpool will beat Man United in
the third week of the Premier League, let alone Liverpool in the previous two games failed to win, then Liverpool will be much more motivated to get
their first win of the season, in order to keep the points difference with Man City not too far away. After all, Man United as an opponent that Liverpool
will face is not in good condition. So the mentality of Man United players is in a slump after suffering two consecutive defeats. Liverpool will take
advantage of this to be able to win.