The Red Devils suffered a 1-0 defeat to West Ham on Sunday, just three days after losing to Brighton.
Martial made an appearance in both games, but he struggled to make an impact.
And fans have grown increasingly frustrated with him as a result.
The Frenchman has received heavy criticism from fans and pundits in recent weeks.
And Shearer, who is the Premier League’s all-time top goalscorer, took a swipe at him following Sunday’s game.
When appearing on Premier League Productions, he said: “Anthony Martial has had enough time at Manchester United.
“You don’t need to make any big decision on him because I think the decision has already been made or it should already have been made.
“He’s been there long enough, he’s had enough chances. He’s not good enough to take Man United to where they want to be.
“If he was good enough he would be starting on a regular basis and he would be scoring goals.
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“He’s had his time and it hasn’t worked for him, that’s my opinion.”
Former Arsenal striker Ian Wright shared a similar view.
He added: “With all due respect to Martial, if you’ve got Wout Weghorst starting in front of you, you’ve got a problem.”
Martial is expected to leave Old Trafford this summer as Erik ten Hag looks to overhaul his squad.
The United boss has reportedly prioritised signing a new striker this summer, with Harry Kane being heavily linked with a move.
Kane is unlikely to sign a new contract with Tottenham and Man Utd are said to be willing to offer him a lucrative deal in an attempt to lure him to the club.