The BBC Sport pundit has criticised the lack of expectation surrounding the Dutchman's Old Trafford tenure.
David Moyes was infamously lambasted during his spell as Manchester United coach before being sacked last season, while Van Gaal, his successor, was given a huge amount of money to spend over the summer.
The result so far has been a mere fourth place in the Premier League - and Shearer has slammed the lack of expectation currently surrounding the multiple title winners.
The pundit also criticised Manchester United's summer signings, none of whom have particularly impressed the former striker so far.
"I've been amazed at the lack of expectation at Manchester United considering the £150 million that Van Gaal has spent," Shearer said during BBC's live broadcast of Cambridge United v Man United in the FA Cup.
"He took over a team with [Wayne] Rooney, [David] De Gea, [Juan] Mata, [Robin] Van Persie and I think it should be the minimum to get into the top four. But, when you look at the criticism other managers have received when they've lost games, I think this guy's got off lightly.
"I don't think any of his signings have been brilliant."
Shearer was speaking ahead of kick-off during Manchester United's FA Cup tie with Cambridge United.
In the Premier League, meanwhile, United have won 11 of their clashes so far this season, drawing seven and losing four. That currently leaves them 12 points off table-toppers Chelsea and there does appear to be a degree of satisfaction both from within and outside the club at that level of achievement (or, as Shearer would likely put it, underachievement).
Manchester United's next league clash comes against Leicester City at Old Trafford. In the reverse fixture, Van Gaal's side lost 5-3 to the Foxes at the King Power Stadium.