It was hard to imagine a more perfect afternoon for Arne Slot – or a more chilling one for Erik ten Hag who, just three games into the Premier League season, is back on familiar ground, with vultures circling and his credentials as Manchester United manager coming under renewed scrutiny.

Slot had won his first two games as Liverpool manager – against Ipswich and Brentford – but this should have been a litmus test. It wasn’t, United were so compliant, their first-half misery epitomised by a dreadful performance from Casemiro. Things didn’t improve after that, the only consolation being that they avoided a real thrashing, one that could rival the 7-0 at Anfield two seasons ago. Liverpool had chances to score a similar number of goals.

A manager can only beat what is in front of him and Slot and his players did so with verve. Their superiority in every area was clear and when Luis Díaz scored his second goal late in the first half to make it 2-0, it was simply a question of the final margin of victory.

They settled for another from Mohamed Salah – the perennial thorn in United’s side – and Slot could look back on more than a fluid attacking performance. His team were ruthless without the ball and it was their third clean sheet in three games. Ultimately, the message was clear: Slot intends to challenge for the title.

Old Trafford was half empty when the club was full, those who remained too stunned to joke. The TV cameras had made the obligatory cutaways to a grim-faced Sir Jim Ratcliffe in the stands and United’s minority owner has plenty to think about. As does everyone else connected with the club.

It was loud and raucous at first, the United fans bringing the fire as Ten Hag had demanded, but it wasn’t long before those who had come from Liverpool made themselves heard. And enjoyed it. It was impossible to ignore the inflatable “seven” and “zero” that some of them had brought with them.

Liverpool thought they had scored early, but VAR interference blocked Trent Alexander-Arnold’s finish; Salah was offside when he connected with Díaz’s cutback. The move had been sparked by Ryan Gravenberch’s jet-heel attack from midfield; United’s defence had been carved up – a sign of things to come.

Liverpool were unfazed by the disappointment. They were full of intensity, pressing high, snarling forward to win possession. They took control of the midfield, where United were extremely open. It was easy to worry from the start that Casemiro wasn’t quick enough with the ball. Or didn’t have enough of it. Unfortunately for United’s controlling midfielder, the first half was a test for him.

The breakthrough came from a misplaced pass from Casemiro and a blistering counterattack from Liverpool; it was Slot’s blueprint throughout the game. The excellent Gravenberch found Salah and when he crossed, Liverpool were left with two men at the far post. United certainly didn’t leave them with three, as they had done the previous weekend at Brighton when they conceded the winning goal in the final minute. Dominik Szoboszlai couldn’t reach Salah’s cross. Díaz could.

Casemiro had followed Alexis Mac Allister high and dangerously in the 11th minute and was lucky to avoid a yellow card; it was the only chance he had. There were almost too many terrible moments from him to mention, including losing his balance and the ball on the touchline, heavy touches and lots of loose passes.

Ten Hag sent Toby Collyer out to warm up late in the first half – the 20-year-old replaced Casemiro for the second period – and it was 2-0. When Casemiro hesitated with the ball, Diaz robbed him and Liverpool were clear again. They worked the ball to Salah, who crossed for Diaz. The finish was low and deadly.

Liverpool might have had more before the break. Szoboszlai was unable to connect cleanly with Andy Robertson’s cross, while full debutant Matthijs de Ligt made a one-on-one challenge to deny Diogo Jota. United’s only first-half fumble came from a Noussair Mazraoui shot that Alisson saved.

Liverpool were keen to ratchet up the pain. The dreaded no-go zone between United’s defence and midfield was back and when Liverpool pushed into it, especially on the transitions, the sirens sounded.

The third goal came after Mac Allister robbed Kobbie Mainoo and Liverpool could smell blood again as United’s defence was lacking. Szoboszlai played Salah in to score and the visitors could have probably added two more straight away, but United looked vulnerable as they often do after conceding a goal. Lisandro Martínez gave the ball away and was lucky that Salah lifted high, while the Liverpool forward also saw a curling shot go past the far post.

United were staring at complete humiliation and Old Trafford was a fearsome place, with boos when Ten Hag replaced Alejandro Garnacho with Amad Diallo. It felt like the fans wanted him to pull the ineffective Marcus Rashford. In all honesty, Ten Hag could have signed anyone.

United did have chances in the second half. Joshua Zirkzee – another debutant – came close three times, Alisson made a couple of decent saves and they looked safer towards the end when Harry Maguire came on for De Ligt. More revealing, however, was Szoboszlai’s 79th-minute miss, when he simply tried to be too clever. And that was no wonder, because it had been too easy for his team.