UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a contest where the approaches didn't complement each other, with excessive holding spoiling the action.
Two judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They prevail," the victor commented in the ringside as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has stalled in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who operated with a tight guard to barely take the better periods.
The spectators' acceptance diminished amid constant clinching, with Parker going down multiple occasions during clinches.
Parker ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight title.
The stadium fell silent for a profoundly moving homage ahead of the contest as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the first major televised match card in the city since Hatton's death. As the last bell faded, the silence gave way to a shout of "there's just one Hatton".
The competition during the bout equaled the emotion. Arthur was sent to the canvas by a left hook squarely on the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured soon, noticeable damage forming under his face as the duo landed hard punches.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the result with scoring tallies of even score, 115-113 and 115-112, but each competitor were persistent and courageous, exactly what their idol Hatton stood for.
