Never mind the protests outside Old Trafford.
There would have been a riot inside the stadium had fans been allowed in, after Manchester United's lamentable display against their fiercest rivals.
For United's capitulation revived Liverpool's top four hopes, Jurgen Klopp's side now knowing wins in their final three games should be enough to secure Champions League football next season.
For United, a second home defeat inside 48 hours was a brutal reminder that, for all their progress this season, they are still some way short of where they need to be if they are to push for the title.
Given this was United's strongest-possible line-up, bar injured skipper Harry Maguire, the defeat reinforced Ole Gunnar Solskjaer's belief he needs at least three major signings this summer.
United's loss also served as a warning that Liverpool, after a campaign blighted by injuries to key personnel, are certain to be stronger next season and surely back challenging for the title.
Having played a reserve side two days earlier in a 2-1 defeat to Leicester, Solskjaer shuffled his pack again – making 10 changes – for United's third Premier League game in five days.
The fixture congestion was caused by the postponement of this game on May 2, after anti-Glazer protestors stormed Old Trafford and got onto the pitch.
There were protests again last night, with around 2,000 fans outside, but a huge police and security presence ensured the game went ahead.
United should have taken the lead early on, a sloppy pass from Alisson intercepted by Edinson Cavani, who shot wide with the goal at his mercy.
But United did go ahead in the 10th minute, Aaron Wan-Bissaka cutting the ball back to Bruno Fernandes, whose shot was deflected into his own net by Nat Phillips.
The visitors were awarded a penalty in the 27th minute, for a perceived foul by Eric Bailly on Phillips, ref Anthony Taylor pointing to the spot.
But after VAR suggested he take another look at the incident, he reversed his decision after seeing that Bailly clearly took the ball first.
Dean Henderson, back in the United line-up, made a fine one-handed save at full stretch, to keep out a goal-bound Diogo Jota effort.
But from the resulting corner, Liverpool were level, Unitd failing to deal with the set-piece, Phillips atoning by steering a shot goalwards, Jota's clever flick sending it over the line.
The visitors took the lead in first-half added time, Trent Alexander-Arnold's majestic free-kick met by Firmino, who planted a header beyond Henderson, after Paul Pogba failed to track him.
It was 3-1 two minutes after the restart, United again the architects of their own downfall, Fred giving the ball away, then Luke Shaw doing the same after it was retrieved.
Alexander-Arnold's shot was spilled by Henderson, allowing Firmino to seize upon the rebound and plunder his second of the night.
Jota should have made it 4-1 on the hour, after Fred conceded possession again, but he thudded his effort against a post.
United pulled a goal back in the 67th minute through Marcus Rashford, with sub Mason Greenwood denied by a goal-line clearance from Phillips.
But Mo Salah made sure of the win in added time with a composed finish after finding himself one-on-one with Henderson, whose woeful positioning summed up United's wretched night.
