The former Hurler of the Year appeared midway through the second half of Kilkenny's incredible extra-time victory over Waterford, a win which set up an All-Ireland quarter-final with Cork in two weeks' time.
Yet the Ballyhale Shamrocks powerhouse insists his fitness remains a serious problem after a longer-than-expected recovery time from an ankle injury picked up in a club game earlier this summer.
"I have been off training the last eight or nine weeks, not doing a whole lot to be honest," Fennelly explained.
"I was doing a bit of swimming, probably stopped that the last two or three weeks there.
"But that's all I could do, it is a lot different to running around a hurling field and lads hitting off you.
"Hopefully this week I will be able to get a bit of training done and build up the fitness but obviously time is ticking."
Asked as to how much involvement he is likely to have against Cork, Fennelly responded: "It depends this week on how my ankle responds. Another aspect of it is fitness. Would I last 70 minutes or would I be better coming on? Over the next 12 days it will be decided.
Drop
"I'm not training. Any player that doesn't train for two weeks, your fitness does actually drop quite quickly after two or three weeks.
"It's been two-plus months for myself so I need to get training, I need to get games into me and obviously time is not there so it's whatever I can do in the mean time just do it and get the head right and do what you can.
"Ligaments and tendons on my left and a bit of bone bruising. Ligaments on my right. The tendon seems to be causing a bit of a problem but it's settling."
Fennelly explained that his club and county team-mate, Henry Shefflin, was in a similar boat in his own recovery from a foot injury after coming on, and then being replaced, last Saturday.
"Obviously, it would be better for him to get a break for longer and get some training under his belt. He obviously hasn't had any training either so obviously this week will be a big help for him. Obviously a few weeks training would be better for him, but it's not there and you just have to do what you can do."