Wor Flags had a 62-word message for owner Mike Ashley before last night's home game against Burnley.
Supporters unfurled a quote from former manager Kevin Keegan.
It read: "Don’t ever give up on your club. Keep supporting it, it’s your club and trust me, one day you will get your club back and it will be everything you wanted it to be.
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"Newcastle United is bigger than anyone. It hurts I know, but just keep going. He is only one man we are a city, a whole population. Trust me.”
The wording of the banner had not been approved by the club, which has encouraged the flag displays.
Newcastle completed their transfer window business after the Premier League game, which ended 1-1.
The club signed three players – Islam Slimani, Kenedy and Martin Dubravka – on loan.
Shearer, the club's all-time record goalscorer, reacted to the display and United's transfer business.
Before those deals were confirmed, Shearer tweeted: "So over 11 seasons @NUFC net spend has been £4.5m a season.
"Throw in the TV money and ever present gate money... One hour left to spend some and give them a chance! Well done to the fans tonight for showing their love for the club. Shame others don't share it."