Against Leicester City, United were beaten 2-1 with the likes of Amad Diallo, Anthony Elanga and Brandon Williams starting and victory for the Foxes has seen them take control in the top four race - to the annoyance of West Ham and Liverpool.
"Without being disrespectful to the players that played tonight, they made ten changes, this is their 'B' team," Sinclair told talkSPORT.
"I think that's disrespectful and it takes away from the top four race.
"I think there needs to be a punishment for Man United. Three points isn't enough because that doesn't affect anything.
"I honestly feel the way they put out this team, the way they didn't safeguard their stadium, I think a six-point deduction should be given to Manchester United."