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.
Obviously Klopp can use Firmino to replace Nunez for a while, but I think he can also use other players like Jota or an attacking midfielder to play as a striker or winger by shifting Diaz to a centre-forward. I think Liverpool will have no problem with that, so it will be handled well by Klopp.