Walker's stunning solo goal set his new club Aston Villa on the road to victory at Bramall Lane and highlighted the price of failure to win promotion back to the Premier League.
The Sheffield-born defender would probably still be wearing red and white stripes if the Blades had only beaten Burnley in the Championship play-off final two seasons ago.
Instead, he was sold to Tottenham Hotspur for 3m and now, after landing a loan move to Villa for the rest of the season, his career is about to rocket.
A lifelong Blades supporter, Walker, still only 20, is fast maturing into a top class footballer. Having spent the first part of this season helping Queens Park Rangers storm the Championship, he now has an even bigger opportunity with Villa. The deal could yet serve as the final part of his 'apprenticeship' and don't be surprised if Walker is even playing for the full England team inside the next 12 months.
Speaking after Saturday's game, Walker admitted that it had been a day of 'mixed emotions.' He refused to celebrate his goal and was applauded off the pitch at the final whistle by all sides of the ground.
"This is actually the first Cup game I've played at Bramall Lane," he said. "And I was a ballboy when United beat Villa here in the Cup six years ago.
"It was nice to play and score this time but I was never going to celebrate. My mum is a United fan and she had told me not to celebrate if we scored.
"I didn't go in the wrong dressing room or anything like that," he added. "But, to be fair, in the first 10 minutes I had to keep saying to myself 'pass to a Villa player rather than a red and white shirt'. I was with this club from the age of seven and, whenever I'm out and around the city centre, the United fans always come up and ask how I am. I don't have a bad word to say about them and I just hope they can get a few results and get themselves safe."
Employed at right-back, Walker burst forward nine minutes into the game, evading two challenges before coolly slotting the ball home from the edge of the penalty area. It was the first goal of his professional career but is unlikely to be the last.
Striker Jamie Ward might have equalised for the Blades soon after but his close-range effort struck the legs of goalkeeper Brad Friedel. Ward was caught up in a similar 'love triangle' to Walker – being a lifelong Villa supporter who now plays for the Blades.
Things got worse for the hosts on 33 minutes when Marc Albrighton volleyed in a cross from Ashley Young to put Villa 2-0 ahead. However, Steve Simonsen's brilliant one-handed save to deny England international Gabby Agbonlahor just before the break kept the Blades in the contest.
Carlos Cuellar's clumsy challenge on Lee Williamson allowed Ward to make it 2-1 from the penalty spot. Buoyed by the goal, the Blades briefly started to push Villa back.
Striker Daniel Bogdanovic nearly punished a rare loose pass by Walker but his 25-yard drive was tipped around the post by Friedel.
The difference in class between the two sides was far from apparent and although new Blades manager Micky Adams was disappointed by the result, there were still a number of positives to take from his side's performance. His squad has been badly affected by injuries and there was only one senior player available to sit on the bench alongside five teenagers.
With 20 minutes to go, Villa perked up after operating in cruise control. Substitute Stilyan Petrov wasted a golden opportunity when one-on-one with Simonsen and went close again, moments later, with a header from a corner.
The dismissal of Ashley Young for a second bookable challenge reduced Villa to 10 men for the final 14 minutes but the Blades were unable to grab an equaliser.
Friedel pushed an overhead kick by Johnny Ertl around the post and a push by Walker on Ward in the penalty area was missed by referee Michael Jones.
Petrov wrapped things up for Villa with a stunning 25-yard strike two minutes into stoppage time.
Only 16,888 attended the game, the lowest of the season so far at Bramall Lane, but it was just another example of how football supporters are cutting their cloths accordingly during these troubled financial times.
When the Blades next line-up at home, against Norwich City in 12 days' time, the gate is firmly expected to be back near the club's 21,000 average.
Victory lifted the pressure on Villa manager Gerard Houllier after just five wins in his first 19 games in charge.
"Whatever happened in this game, I do not think there would have been a major change," he insisted.
Commenting on the departure of Roy Hodgson and short term appointment of Kenny Dalglish as manager at his former club Liverpool, Houllier said: "This world has become brutal.
"Roy was 'manager of the year' last season and is still a great manager now. I feel for him although I like Kenny and we have been friends for a long time.
"I think Kenny will see a change in the game," Houllier added. "There is no time for patience now. You lose two games and you are in the firing line."
Hero: Kyle Walker
The former Blades defender 'celebrated' his debut for Aston Villa by scoring his first senior goal but refused to celebrate. A lifelong Blades supporter who was sold to Tottenham for 3m two summers ago, Walker was applauded off the field at the final whistle by all sides of Bramall Lane.
Villain: Ashley Young
After being booked for a blatant pull on the shirt of Blades defender Elian Parrino, the Villa and England winger stormed down the tunnel after being sent off for a second bookable foul with the score at 2-1 and 14 minutes remaining.
9 mins: Walker's goal. After knocking the ball past Jamie Ward on the halfway line, the Villa full-back surged forward and skipped another challenge before tucking the ball home from the edge of the box.
Michael Jones: Took no nonsense from Villa and sent off Ashley Young for two bookable offences.
Three games without a win for new Blades manager Micky Adams but still promising signs in terms of passion and commitment and no disgrace in losing to Aston Villa.
Quote of the day
My mum is a United fan and she had told me not to celebrate if we scored!
– Kyle Walker revealing that he was under strict family orders on his return to Bramall Lane.
Coventry City, away, Saturday January 15, Championship.
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