The Magpies, 11th in the Premier League with only one win in 2015, conceded a penalty inside 30 seconds and were 3-0 down after 21 minutes.
Two further goals condemned Newcastle to a 5-0 defeat, their heaviest since a 6-0 loss against Liverpool in April 2013.
Newcastle have one just one of eight games since John Carver took temporary charge following Alan Pardew's departure to Crystal Palace last month.
"They were pathetic. It was embarrassing, from the first minute to the last," Shearer, who scored 206 goals in 10 seasons for Newcastle, said on BBC Match of the Day.
"I can just imagine the Newcastle team talk before the game; 'Right lads, this team have got Barcelona (in the Champions League) on Tuesday, let's just keep it tight for the first 10 or 15 minutes. Let's get in among them, put one or two tackles in and see if they fancy it.'
"But after 20 or 30 seconds, that is out of the window. Newcastle just didn't get near them. It was just far, far too easy for City. These are top players. You cannot give them time on the ball like that.
"City showed so much hunger to get the ball back, even when they're 3-0 up. I never saw that once from Newcastle. It was pathetic."
Newcastle host relegation-threatened Aston Villa at St James' Park on Saturday.
(Reporting By Tom Hayward; Editing by Amlan Chakraborty)
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