The 20-year-old, who made his first-team debut as a substitute three years ago at Swansea, slotted in at centre-back following an injury to Harry Maguire and put in an accomplished display as the Blades made it eight wins on the bounce.
It was his first appearance at Bramall Lane and after only finding out shortly before kick-off that he would be starting, Kennedy reckons it went as well as he could have hoped.
"It is a dream to play at Bramall Lane to make my full debut and I'm happy to get three points and a clean sheet," he said.
"About an hour before kick-off I came into the dressing room, saw my name on the teamsheet and just took it from there.
"I'll never forget that moment, I have been making the squad a lot recently and I knew I had half a chance of playing, so I had butterflies all day and as soon as I saw my name I was buzzing.
"It was the best moment of my career. Making my debut at 17 was good, but we got battered that day, so coming in and winning was great.
"It couldn't have gone much better, to keep a clean sheet as a defender you can't do more than that."
Kennedy put himself in the frame for a start in Sunday's FA Cup quarter-final against Charlton, though he expects to lose out to the more established Maguire, who will return from injury.
"I'm just happy to be in the squad and playing football, it is up to the gaffer who he picks," he added.
"Harry has had a great season, so it would be no shame in losing out to him. You never know, but even if I get called up just for 10 minutes I'd be happy to do it.
"I'm going to work hard until the end of the season. My contract is up at the end of the season, so I'm just going to keep working hard and keep my head down and see where it takes me."
Boss Nigel Clough, who hinted at a recall for Maguire, was still full of praise for Kennedy's performance.
"He has been exceptional in the Under-21s, he has been on the bench for us most of the time this season because Maguire and (Neill) Collins have been so good, but when we needed him, he came through," said Clough.
"He was great, he has been the stand-out player in the Under-21s so I had no hesitation at all in starting him.
"He has a lot of character, when you haven't had too many matches and come in and do as well as he did, the word whole-hearted doesn't do him justice."
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