The Australian hooker had only joined the club the previous day, escaping his frustrating lack of game-time at Leeds Rhinos where he won a Grand Final just two years ago.
Yet when he scored in the 79th minute to finally settle an absorbing and at times frenetic derby, all that frustration must have been released in an instant.
Parcell produced a lovely runaround with prop Mitch Garbutt - his former Leeds team-mate who was colossal in underpinning this crucial victory - and burrowed over to secure the win that sees Rovers rise off the bottom of Super League.
Until then, Marc Sneyd’s missed conversion attempt for Hull had looked like being the decisive factor.
The prolific goalkicker is rarely offcue with his kicks - especially those as standard as the one to improve Andre Savelio’s 65th minute try which left it at 12-10 - but, perhaps fittingly, this was a game of missed opportunities.
Chances came and went for both, but particularly for KR and, so, they can be no arguments that they eventually came away with the two points.
Hull, up to third with six wins in their previous seven outings but missing Albert Kelly here, had inflicted a record 56-12 derby defeat on KR when the sides last met on Good Friday.
However, this Robins side is unrecognisable to that one; there is plenty of work still to do with nine rounds to go but clearly Smith has them heading in the right direction.
It is the second win in four games since the former England coach replaced Tim Sheens.
Mose Masoe was superb up front with Garbutt and Parcell - despite just one training session - brought all the requisite class and direction at dummy-half that Smith had envisaged when swapping him with Shaun Lunt.
Rovers had taken a 12-6 lead when captain Joel Tomkins raced onto Josh Drinkwater’s clever grubber for Ryan Shaw to convert in the 50th minute.
It was the least his side deserved. They had started the second period far better than their opponents.
It was not due to anything spectacular. They just ran harder and with more purpose and controlled the ball far better.
It was Smith making his mark. He always said pure desire could be the deciding factor and his side seemed to have more last night.
Still, their profligacy almost came back to haunt them; they should have scored directly after Tomkins’ effort when Craig Hall raced down the right touchline only to try the more difficult pass - further infield to Josh Drinkwater rather than the nearer Danny McGuire - and the ball went to ground.
Typically, Savelio then crossed with Hull’s first real chance of the second half, hitting a fine line off Jake Connor’s pass.
Lee Radford’s team had looked flustered and at odds with each other but - despite Sneyd’s miss - still had chances.
Rovers had worked too hard, though, to let this one slip and they held on before Parcell wrapped things up.
Anyone seeing the 6-6 interval scoreline must have thought defences were thoroughly on top in a bruising, tight derby with little sign of breakthrough.
They would be wrong. There was a plentiful supply of chances for both sides but neither could make the most of them.
Both were so wasteful. Such is Hull’s form and the strike in their squad, you sensed they would make up for it in the second period but they were simply not allowed to.
Smith must have been worried at the interval, though, given how many of his side’s well-crafted plays slipped away to nothing.
Ben Crooks had seen them level in the 15th minute after a brilliant pick up following Adam Quinlan’s superb break and chip ahead.
That came off their first penalty and showed what they were capable of but they could not add to that score.
Crooks got through the line again but spilled in the process and there were three more clear-cut chances.
Rovers fans must have been wondering what Parcell - renowned for his running game - was bringing to the contest when he did not raid once in the opening quarter.
But that’s what the former Manly player does; waits and waits for his moment and then off he went with his searing acceleration from deep inside his own half in the 27th minute.
Shaw was up in support but spilled when tackled by the covering Ratu Naulago.
Masoe and McGuire both broke, too, each with no finish as colleagues fumbled.
Hull had opened the scoring as early as the sixth minute when Bureta Faraimo collected Sneyd’s lofted chip with ease but then proceeded to mess up every other chance that came their way.
Faraimo, Jake Connor and Sika Manu all butchered chances. On and on it went. Until KR took charge.
Hull KR: Quinlan; Shaw, Linnett, Crooks, Hall; Drinkwater, McGuire; Johnson, Parcell, Murray, Tomkins, Hauraki, Lawler. Substitutes: Garbutt, Masoe, Atkin, Livett.
Hull FC: Shaul; Naulago, Tuimavave, Griffin, Faaimo, ; Connor, Sneyd; Green, Houghton, Paea, Minichiello, Manu, Westerman. Substitutes: Fash, Howden, Taylor, Savelio.
Referee: Robert Hicks (Oldham)