Since Amorim's appointment, United have risen just one place after Ten Hag was sacked in October.
“They're actually worse now than they were under Ten Hag because of what this manager wants to do with his system," Shearer said on The Rest is Football.
“They can't play the way he wants to play. The players just can't do it.”
Amorim’s side were humiliated by Brighton this weekend in a loss that makes their record at home this season the club’s worst in the league in 131 years.
Amorim said afterwards: “We are the worst team maybe in the history of Manchester United. I am saying that because we have to acknowledge that and change it. Here you go: your headlines.
“It’s unacceptable to lose so many games. For any Premier League club, so imagine Manchester United?
“The opponents are better than us in many details. I am just here to help my players, but we need to understand we are breaking all the bad records.
“In 10 games in the Premier League, we won two. I know that. Imagine what this is (like) for a United fan. Imagine what this is (like) for me. We are getting a new coach who is losing more than the last coach. I have full knowledge of that.”