The Portuguese was dismissed for a high challenge on Alvaro Arbeloa as both players contested an aerial ball in the second half of their Champions League last 16 second leg.
United assistant Mike Phelan, in the absence of a seething Sir Alex Ferguson, described the incident as "amazing" to the press afterwards.
However, speaking on ITV's coverage of the match, Keane argued: "I think the referee has actually made the right call.
"Everyone's upset about it and it's slightly unlucky, but it's dangerous play. Whether he meant it or not is irrelevant. It's dangerous play - it's a red card.
"You have to be aware of other players on the pitch. Does he think he's going to have 20 yards to himself?
"Whether it's [a brave decision] or not, it's the right decision. Whether he meant it or not doesn't matter.
"Nani's a quick boy to go down anyway. He's not the bravest player on the planet."
The Irishman says the Turkish official had carefully thought out his decision and reckons that the referee's assistant may have had an influence too.
He added: "It doesn't matter if he knows there's somebody there or not, it's irrelevant. I don't think the referee actually made the decision because he waited a couple of minutes while the player was getting treatment.
"I think it might have been the assistant that made the decision, and I actually think he's made the right call.
"We always say referees are very quick to make decisions - he waited a couple of minutes.
"Any time I was sent off in my career I always thought, 'did I give the referee chance to send me off?' And if the answer is yes, then it's out of your hands."