Keane was sent off for a last-minute challenge on his former Republic of Ireland team-mate as United were held to a 1-1 at Sunderland on Saturday.
Ferguson jumped to the defence of his controversial captain after the game, calling it a "soft" dismissal and an "innocuous incident."
But after reviewing video footage of the incident, Ferguson has admitted referee Uriah Rennie had no choice but to issue Keane with a red card.
United will not appeal against the sending-off, and Keane is set to be handed a club fine of two weeks' wages, believed to be around £180,000.
"Roy is the first United player to be sent off for this offence and I have said recently the use of the elbow is a growing problem in our game," Ferguson said.
"We won't appeal, the referee had no choice but to send him off.
"The replay I was shown immediately after the match was misleading. When I saw the TV pictures from a different angle, I saw the referee had no choice.
Automatic ban
Ferguson does not believe, however, that Keane's foul was the worst of its kind.
"I still think it was petty rather than grievous," he added.
Keane's red card was the 10th of his United career, and his 11th in total.
He is likely to pick up an automatic three-match ban, while an additional further ban could result from any Football Association disrepute charge over the revelations in his autobiography.
The FA is due to decide early this week whether to bring a charge against the player after his admission that he deliberately set out to injure Manchester City's Alf-Inge Haaland.