Sir Alex Ferguson felt Howard Webb's decision to send off Cristiano Ronaldo for what the referee deemed as deliberate hand-ball was a harsh one.
The 68th minute dismissal of the United winger was the only blot on the Reds' copybook during a crucial 1-0 victory over their local neighbours.
The United boss believed Ronaldo, who now faces a one-match ban, was merely protecting his face when he rose to meet Wayne Rooney's right wing corner.
"I've watched it again and he's trying to protect the ball hitting his face," he told MUTV. "He also thought he heard the whistle.
"It's not as if he's thrown a hand at the ball to deflect it away or punch it towards goal, it simply hit his hands.
"If it had been outside the box, the referee would just have given a free-kick, nothing else. The crowd played their part of course and he got sent off.
"Sometimes you just have to overcome things and we overcame a lot today," added the manager. "Winning with ten men is a difficult task away from home and we did that well today."