The Frenchman signed for Paris Saint-Germain for a reported £20m last month.
While the Magpies tried to recruit Lyon's Clement Grenier as a replacement, the North-East outfit failed to convince the player to make the move to the Premier League.
"If you're going to let your best player go, you have to have a plan for someone to come in," Shearer told talkSPORT. "You can't let him leave and then bring no-one else in. That just can't happen.
"It sends a bad message out to the fans and, if you're a player in that dressing room, you must be thinking, 'where are we going?'"