Mane is nearly irreplaceable in Klopp's scheme, because Klopp's scheme is almost unique. It's the only thing I watch where the midfield doesn't act like classical midfield at all. Attack doesn't come from foot-clever midfielders buffooning opponents with magic mostly down the center and then feeding strikers (as brilliantly illustrated by, say, DeBruyne-to-Aquero), but by wingbacks taking charge of midfield yardage and then finding super-clever strikers also at the wings (fed by them and Firmino --- the other utterly non-classic "9".)
Mane is perfect for this. He's so good getting the ball wide after opponents have already been stretched by Liverpool's wingbacks, he demands double hasty coverage and beats it anyway. Plus he's oddly strong, and fast, and willing to come all the way back to help Robertson's side. He's more willing to pass than Salah (who plays more like a selfish bent-to-the-right "9"), and he and BobbieF have great chemistry.
So what does Liverpool's midfield do? I have been glad to hear the commentators finally waking up to how different and uber-significant they are. This is NOT at all a typical midfield. It doesn't have to be DeBruyne-level (Modric, Silva, Iniesta et al) clever, nor borderline roughhouse dirty like some others (though Fabinho has a tinge of that.) Liverpool's midfield is Henderson and Wynaldum and Milner-like. They are made for whole game brave combat. They are the werewolves of what should become the modern game. They throw themselves at the ball wherever it's flying. (Sort of like Arturo Vidal when he's behaving himself.) "Work Rate"out of the Chart.
Someone asked USWNT team star Julie Ertz how she wanted to play. "Like a Calm Maniac," she said. THAT's Henderson and Wynaldum ... and Fabinho didn't get it at first, but is now on board. Firmino and Milner already had the genes for it. Mane has now got it too. (Salah doesn't.) People gushed over the signing of Nabi Keitha ("What a talent!") Well, yes --- but he DOESN'T get the needed Liverpool New Era Midfield style. Shakiri has tried but is largely lost running around ineffectively. Lallana gets it. Oxlaide-Chamberlain seems to WANT to do this, but doesn't (yet) have the Henderson/Wynaldum instinct. You could bring GREAT Old-Style midfield talent into Liverpool and actually severely hurt this team.
Mane and Firmino are the perfect forwards for this Chaos-creating team style. Feast off the midfield aggression's turnovers and the never-slow-down wingbacks pressures. Fury on the field from line to line and mostly in the opponents' side. And when the fury strikes, you need a finisher who both strikes and passes, because the rest of the Fast and Furious are still moving forward. Mane: Don't get hurt.