Kelly has been out since picking up an ankle injury on his full debut last month. He was hoping to return this week but a fresh scan on Sunday has revealed another strain in his leg and he faces at least another week out.
"I came back but in my first training session I knocked it again. I went for a scan on Sunday and it showed I have a little strain on my ankle. It's another seven to 10 days," said the defender.
"It's a setback but I've just got to work hard with the physios to try and get back as soon as possible.
"As a footballer you are going to get injuries. The thing that matters is how you come back from them.
"Obviously I was gutted with the timing of it. There's been a few injuries around the first team so I could have got another chance but I'm still young and hopefully when I'm back I'll have more chances to show what I can do."