Alexander Isak has been left out of Newcastle‘s travelling squad for their pre-season tour of Singapore and South Korea, amid ongoing interest from Liverpool.Newcastle announced a 30-man squad for their trip to Asia on Thursday morning, ahead of friendlies against Arsenal, Tottenham and the K-League All-Stars.But Isak has not joined his teammates on the flight, with the club claiming this is due to a minor thigh injury.The Swedish striker was also left out of Newcastle‘s opening friendly of pre-season on Saturday – a 4-0 loss to Celtic – with manager Eddie Howe later admitting this was due to speculation over his future.Despite confirming the £79 million signing of Hugo Ekitike on Wednesday evening, Liverpool “retain an interest” in Isak according to the Times‘ Paul Joyce.While Newcastle are adamant their centre-forward is not for sale following an informal proposal in the region of £120 million, there is a sense that his head may have been turned by overtures from Anfield.Isak trained separately from the rest of the Newcastle squad earlier in the week and there remains a possibility that he will push for a move to Liverpool.If so, the money is still there to sanction a deal that could cost as much as £150 million.Sky Sports have relayed research from football finance expert Swiss Ramble in explaining that Liverpool “can lose a further £158 million and still be [PSR] compliant next summer.”That seemingly does not factor in further sales, with Darwin Nunez and Luis Diaz among the most high-profile candidates for a departure before September 1.Liverpool have already spent £295 million on seven players this summer but Joyce writes that their incomings are “set to continue.”There is a theory that both Ekitike and Isak could be accommodated in the same squad – that, after all, was Newcastle‘s intention when they bid for the Frenchman – and information around the new signing supports that. Joyce writes: “[Arne] Slot views the player as a multi-functional centre-forward who, while primarily playing as a No. 9, can also operate wide or perform as a second striker.”It cannot be overlooked that Liverpool lost a player in that mould to tragic circumstances this summer following the death of Diogo Jota, and any moves for Nunez, Diaz or both would require top-level replacements too.It is a story that could develop in the coming days, with it doubtful Newcastle would have left Isak on Tyneside for a minor injury, having included left-back Lewis Hall who is still recovering after months out himself.