Isak has made a transfer request to Newcastle this week amid intense interest from Liverpool.
The Swede was not on the jet for the squad's flight to Asia for the beginning of their preseason tour with management insisting he had been left behind to nurse a thigh injury.
However, it soon emerged Isak had made a transfer request leaving fans stunned.
Former Newcastle captain Shearer told Betfair: "It's ridiculous what Newcastle put out this morning to say he wasn't on the trip because of a slight thigh injury. If they thought people wouldn't see through that… it was disappointing. They should've just told the truth straight away. I understand it's a very difficult situation for them but it is what it is.
"Eddie Howe has to use all his persuasive powers to do what he can and try to make him stay at least one more year. If that can't happen then it is what it is. You have to get the very best deal for the football club and if someone is prepared to pay north of £150million, and if he really, really wants to go, you can't step in his way.
"Isak has been brilliant for Newcastle, helped us achieve our dream of winning a trophy and if there's no way whatsoever that he can be persuaded to stay at Newcastle, then they have to get the best deal possible and if someone's prepared to pay what they want, you have to say thank you and build for the future. There's no individual bigger than the football club."