The West Ham striker courted controversy in the 45th minute of the 2-2 draw at Upton Park when he clattered into the Spanish keeper from a corner.
Though he was later booked for another firm challenge on De Gea, Ferguson felt Carroll should have seen red for the incident at the end of the first half.
“I think it's obvious," Ferguson told Sky Sports. "I don't think you dwell on that... it's an obvious red card but the referee has seen it differently.
”We know how they [West Ham] play, the ball is in the air most of the time and you have to defend those things. They are very, very aggressive, as we have seen, you hope there's a strong referee and I am not so sure we got that.
“David has improved all the time all season and he is developing into a fantastic goalkeeper.
"It's experience of the English game. He is getting strong and his training performances are really good. He's shown that again.
"We have been saying for a while now he has developed into a fantastic goalkeeper.”
West Ham manager Sam Allardyce defended Carroll and said the striker's momentum took him into the United keeper.
“You cannot commit on a corner and set off running and then even when you are not going to get the ball stop dead," Allardyce said.
"As you know when you are travelling in your car at 60 mph it takes you 300 yards to stop. With Andy going at 15mph he can't stop dead. It was never a red card for me, a yellow perhaps.”
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