Duff suffered a calf strain just 30 minutes into the new season at home to Nottingham Forest and was replaced by Carlisle, who has since excelled in the centre of defence.
But Carlisle was harshly dismissed just seven minutes into Saturday's trip to Millwall, which meant Duff was thrust back into action.
Despite battling with cramp during the closing stages, the Northern Ireland international helped 10-man Burnley claim a 1-1 draw and hopes to cement a starting role during Carlisle's absence.
"This is what you wait for," Duff told the club's official website.
"It's disappointing when it's someone else's misfortune but that's how it works.
"Clarkey has taken his opportunity well when I got injured and this is what a squad and competition is all about.
"Clarkey might be suspended for three games now so hopefully I will get in the team and kick on from there again."
Burnley have appealed against Carlisle's red card, although his ban could be extended if the appeal is unsuccessful.
Duff, 32, is set to start at Sheffield United after the international break but insists there is a friendly rivalry with Carlisle.
"Me and Clarkey had a laugh about my situation," he added. "I think I've said before he told me he was the one who shot me from the stand.
"He came on after 30 minutes against Forest and, to be fair to Clarkey, he has been one of our best players since he cane back and I had to wish him well.
"You don't wish anybody bad luck or misfortune but I've had an opportunity now. I thought I did okay and hopefully I will get a chance in the next game."