The Swedish striker has been heavily linked with a transfer to Liverpool, who are thought to be ready to pay upwards of £120 million to sign him.
The Magpies have been insistent that Isak would not be for sale this summer, however they could now be forced to change their stance.
The Daily Mail claims Isak now wants to consider his options, with club chiefs aware of his next deliberations after failed contract talks.
The report adds that Isak's representatives are not interested in committing to a new deal at St James' Park with Liverpool still lurking.
It comes after the 25-year-old was left out of Newcastle's squad for a pre-season tour of Asia, which the club put down to a minor thigh injury.
He was also absent from the team's 4-0 defeat at Celtic last week, with Eddie Howe explaining the decision was down to ongoing transfer speculation.
Liverpool have made significant moves already this summer, setting a British transfer record by signing Florian Wirtz from Bayer Leverkusen in a deal totalling £116 million.
Arne Slot's Reds added forward Hugo Ekitike to their ranks on Wednesday, taking their total spend in the window close to £300 million.