The Englishman hurt his ankle in an awkward fall in the second half of the Reds' 1-0 loss to Brisbane Roar and it has been confirmed that he will now miss the rest of the season.
"Joey Keenan is definitely out, he's broken his fibula so he's out for the whole season," said Adelaide coach Rini Coolen.
"To be honest at the end of the day I was more disappointed about that than the loss. I was happy with the performance from my team in the last two games they played.
"If you lose the game you're still happy with the performance of your players. If you lose the game it's not positive but maybe losing a couple of players I have more problems with that."
And what's worse for Adelaide is that Marcos Flores, Cassio and Nigel Boogaard will have MRI scans for various problems as injuries start to hit the second-placed club, who have lost four of their past five matches.
"There will be a lot of MRI scans today and we'll have to find out during the week if we can fit them in the squad for the next game," Coolen added.
"Boogaard will have an MRI today. He was having trouble with his groin but there was a little bit better feeling than there was yesterday so that could be positive.
"And it's the same with Flores and Cassio. They both have problems on their hamstring. It's all about what I said yesterday, the schedule we have.
"There were already problems at half time and we played a real good 45 minutes I think. What I saw at half time was definitely the reason of the schedule of the last couple of weeks but we couldn't change it."