Keenan was sent-off in the ninth minute by Syl Doyle for allegedly punching Down’s Conor Laverty.
However the Tyrone legend believes his man should not have been dismissed.
He explained: “I am bitterly disappointed with that aspect of the game because I don’t think Daryl was even booked during the whole National League.
“He is a very honest footballer and a very clean footballer and he says he was sent off in the wrong, that he didn’t strike.
“It is hard enough to beat a team of Down’s quality with 15 men, let alone 14. I asked them to go out and win the second-half and they were able to do that, they kept battling away so you have to admire their spirit.’’
Fermanagh did not start Seamus Quigley, but Canavan denied reports he was dropped for failing to show up for training last week.
“Seamie hurt his hamstring last Sunday in a training weekend and that’s why he didn’t start.
“Shane (McCabe) was ill with a bug. We hoped to get him for 10, 15 minutes but he wasn’t well and he was honest enough to admit that.’’
James McCartan was relieved to get a first win under his belt but was far from impressed with Down.
“It (Keenan red) was a key moment surely, but anyone who wins an Ulster championship match is happy no matter how ugly it is and it was probably ugly there at times.
I’m not blaming Fermanagh for that – we defend in numbers too. They’re a team you have to break down and we did that. We’re delighted to get a win.’’