The Sweden striker is being linked with Arsenal and Barcelona ahead of the January market.
But Shearer told Betfair: "There have been reports this week that Newcastle would be willing to take £115m for Alexander Isaak in January. For me, it’s a no.
""Unless they have a like-for-like, world class replacement ready to come in then it would be crazy to sell him in January and I don't think they will. It's not a viable option."
On Newcastle's interest in Crystal Palace defender Marc Guehi, he continued: "Clearly there's cash available if they were bidding for Marc Guehi but I don't know if that would've meant selling one of the big-hitters in the summer. I don't like the PSR rules and I don't think they're fair or right but that's the situation we're in.
"Newcastle are having an up and down season, the international break came at the wrong time because they had a bit of momentum, were getting results and they didn't need the break.
"They didn't need players flying here, there and everywhere. Clearly that impacted them against West Ham because after 10/15 minutes, they were nowhere near as good as what they have been in the past month."
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