Newcastle boss Eddie Howe has warned Newcastle they would have to start again if they sold star striker Alexander Isak this summer.

The 24-year-old Sweden international, the Magpies’ £63million record signing, has been linked with both Arsenal and Tottenham in recent weeks amid the perception that the Saudi-backed club may have to trade to comply with profit and sustainability rules.

Isak enhanced his blossoming reputation further with a superb double in a 4-0 Premier League victory over Champions League-chasing Spurs at St James’ Park on Saturday to take his tally for an injury-interrupted season to 21.

Asked about the need to retain his services, head coach Howe said: “We are trying to build a team, we are trying to grow everything, really, upwards and to do that as quickly as possible and as efficiently as you can, you need to keep your best players, otherwise you enter a different period where you go into transition and you have to start again.

“Your top players, the elite ones, are so difficult to recruit, they’re so difficult to find, so when we get them, we’ve got to try to keep them.”

Isak left Micky van de Ven sitting on his backside to open the scoring on the half-hour and when Anthony Gordon did the same two minutes later, the visitors were in trouble.

Isak’s equally accomplished 51st-minute finish put the result beyond doubt and Fabian Schar added the flourish at the death with a thumping header on a day when the Magpies’ game-plan worked to perfection.

For long periods, they were content to allow Spurs possession – they had just a 27 per cent share – and back themselves both to deny them clear-cut chances and to hit them on the break, and Howe could not have asked his players to execute his instructions any more comprehensively.

Asked how good Isak could be, Howe said: “For me, he’s so exciting to work with, he’s got so many facets to his game we can explore and try to make better.

“First and foremost, he has the undeniable quality that he wants to score, he needs to score – that’s a great characteristic for any striker to have.

“But he also plays for the team, he doesn’t play for himself, which is rare. You can see him linking play and doing things that the team needs, not just what he needs. I thought it was a great performance from him today.”

It proved a testing day for Ange Postecoglou’s men, who were trounced 6-1 in the corresponding fixture last season before his arrival, although the Australian played down the significance of that result.

Postecoglou said: “I think there are other explanations beyond that as to why we didn’t perform. Some of it is down to the opposition, some of it is just down to us.

“We didn’t really get a control of the game, like we have been, and we paid a price for that.”