The Manchester United boss insists the QPR midfielder had to be shown a straight red card after bringing down Red Devils winger Ashley Young in the penalty area
A first-half Wayne Rooney penalty and a goal from Paul Scholes ensured United took all three points, but the game was not without controversy.
Rangers midfielder Derry was dismissed after just 14 minutes of the Old Trafford clash after Lee Mason adjudged the 34-year-old had fouled Ashley Young in the area.
Young appeared to be in an offside position when he received a pass from Wayne Rooney, but Ferguson felt the contact in the area was worthy of a penalty.
“I thought Ashley was offside,” the Scot told Sky Sports.
“You get a lot of it, in the last few weeks there has been a lot of decisions like that.
"The boy there just tugged it enough to unbalance Ashley and obviously being the last defender the referee is left with no option.”
And despite the red card giving his side an early advantage, the United manager was not happy with how his side responded.
“The sending off put us in the comfort zone to be honest and I didn't enjoy that part because before the sending off I thought we played really well,” he said.
“The speed of our play was terrific, switches of play, good movement. The sending off put us in that comfort zone and we got far too casual.”
Despite being unimpressed with his team’s performance, Ferguson reserved special praise for the evergreen Scholes.
“We've only dropped two points since he came back into the fold, and today was a typical Paul Scholes goal," he added.
“He's scored three now since he's come back, you see the control of the game he has and the control of the tempo of our game, that's never changed really.
"He's a great, great player.”
The Scot wouldn't be drawn into commenting on how Sunday’s win affects the title-race, with his side stretching their lead over rivals Manchester City.
“The only thing I can take out of today is that we won our game,” he said.
“There's only six games left now and we're a goal better off than City, so that's the only way I can look at it.”