Anthony Elanga is the only outfield player through the door at St James' Park so far this summer, and Isak wants out after Liverpool's approach.
In an ideal world, Shearer would want to keep Isak but says if that's not possible the Magpies must make sure they source a replacement before allowing their star man to move on.
“From a Newcastle point of view, it's been a really disappointing summer up to now,” said Shearer.
“It would be ideal \[if Isak stayed\], but I'm not naive enough to know that these things happen in football, it's going on with Newcastle and it's not a nice situation for him to deal with.
“But if it's impossible to talk him around and he hasn't made his mind up, then they have to get someone in before they can let him go.
“So it's a matter of sitting and waiting and sitting. We don't know the full picture.
“Having said that, they’ve signed Anthony Elanga and I’m pretty sure they’ll get more in.”
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Shearer’s former club, for whom he scored 148 Premier League goals, had hoped to replace Isak with RB Leipzig’s Benjamin Sesko but are set to be beaten to that signing by Manchester United.
After leaving Blackburn Rovers in 1996, Shearer had been faced with the same choice between the Magpies and the Red Devils but made the opposite decision to Sesko.
The Premier League legend, however, does not blame the 22-year-old for his choice to move to Manchester.
“That's his choice. He's chosen to go to a club who finished 15th last year and are not in Europe this year,” he said.
“It's a lot of money. I watched him a few times last year. There's definitely room for improvement, which I'm sure he will."
The all-time Premier League record goalscorer was speaking at the launch of the new Topps Premier League 2026 which features players from all 20 clubs in the league in their 2025/26 kits at the Fanatics Collectibles flagship store on Thursday .
He was joined by other legends such as Steven Gerrard and Rio Ferdinand to celebrate the new stand-alone range that features legends and current stars.
_**Marking the launch of the Topps Premier League 2025-26 collection were football icons Alan Shearer, Steven Gerrard, and Rio Ferdinand, joined by Jermaine Defoe, Joleon Lescott, Theo Walcott and Tosin Adarabioyo, alongside Fanatics CEO Michael Rubin in a star-studded celebration.**_