The former Newcastle and England striker said the club needed to strengthen after a successful year to cement their status as European challengers, and Ashley should have given Alan Pardew more backing in the transfer window.
"Newcastle missed a huge opportunity in the summer," told The Sun.
"Having punched above their weight last season and almost made it into the top four, this was the time to spend.
"The club was up and running again after so many years mired in uncertainty. There was once again a feel-good factor and even Mike Ashley was enjoying it.
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Shearer also stated that while he does not expect Newcastle to match the spending power of Chelsea and the two Manchester clubs, they need to match clubs like Tottenham to be able compete on the pitch for a place in the top four.
"It's up to Ashley how much he spends — but just look at what Spurs did," he added.
"They pipped Newcastle to fourth place and went out and spent £57 million on six players. Newcastle spent £8.1m on three and got a fourth in on a free.
"Nowadays in the Premier League you need to spend £15m just to stand still.
"Newcastle needed to move forward again, particularly with the demands of the Europa League factored into the equation.
"They should be knocking on the door of the top four and occasionally getting in it."