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The Magpies were booed off following the 3-1 loss to Bournemouth on Saturday - a third successive league defeat that leaves them 19th in the table.
It also increased speculation over head coach Steve McClaren's future.
"If that's a performance from the players to say they are backing the manager, they are in trouble," Shearer told BBC's Match of the Day.
"McClaren is in massive trouble, he has to take his fair share of the blame."
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Shearer, the Magpies' all-time leading goalscorer, was in temporary charge of the club the last time they were relegated in 2009.
"There is no statement of intent on the pitch from the players, and there are no academy players coming through," he added.
"Chief scout Graham Carr holds most of the power when it comes to players coming in and out. There was a £15m striker [Aleksandar Mitrovic] who sat on the bench today."
Newcastle are currently a point behind 17th-place Sunderland with a game in hand, but they are nine points behind Swansea in 16th, with 10 league games remaining.
McClaren admitted after the game that his side were now in a "four-team league" battling against relegation along with Sunderland, Norwich and bottom club Aston Villa.
He also said their defeat on Saturday was "going down material". It is now only one win in their last five games, in which they have conceded 12 goals in losses to Everton, Chelsea, Stoke and Bournemouth.
Former Arsenal defender Martin Keown played under McClaren when he was a coach in the England set-up in the early 2000s and questioned his ability to lead a team.
"I think McClaren is an excellent coach but not a manager. He looked like a rabbit caught in the headlights," he told BBC's Match of the Day.
"With managers like David Moyes, Brendan Rodgers and Rafael Benitez around, he's in trouble."
Newcastle are "a mess from top to bottom", according to former captain Alan Shearer.