Kasey was brought in as Sully's deputy this summer from Spanish outfit Rayo Vallecano and has waited patiently for his opportunity.
That chance finally arrived at Villa Park on Saturday and our American international performed admirably until a last-second penalty ruined his and the team's afternoon.
Glenn explained: "It was a difficult decision I had to make, it really was.
"Sully's been terrific for us but I just felt Kasey's been waiting in the wings and it was the right time to maybe throw him in and see what he could do for us. He played very well.
"When you build a squad you are going to use them during a season. Players will be used through injury, lack of form, tactical changes and that's no different for a goalkeeper.
"Kasey's come in and has been very unlucky not to keep a clean sheet. Without the penalty I think he would have had one."
Asked how difficult the goalkeeping situation would be in the coming weeks, Glenn responded: "I get paid to win matches, or to try to win matches and that's the major thing. The team comes first.
"Sully is a tremendous keeper and at his very best, one of, if not the best British keeper.
"Outfield players sometimes come down from their form a little and get rested or left out. I think in the long run it might do Sully some good."
Glenn Hoddle said he felt the time was right to hand Kasey Keller his chance - and insisted the break might do Neil Sullivan good in the long run.