British light-heavyweight Joshua Buatsi made a winning yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a match where the techniques didn't complement each other, with constant grabbing disrupting the contest.
The scoring judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my best showing but what defines champions? They win," the victor said in the ringside as he was greeted with boos from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Various analysts felt the challenger, 31, merited the victory but the contender suffered the second loss of his 28-bout professional journey.
The Champion Secures Win in Slow Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the better rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker falling to the floor various instances during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Hatton Homage Fuels All-Manchester Contest
In the co-main event, Arthur defeated Rea by split-decision in a terrific all-Manchester contest to capture the continental belt.
The arena became quiet for a extremely emotional tribute prior to the bout as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Hatton".
The competition inside the ropes matched the feeling. Arthur was floored by a significant blow flush on the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was injured early, significant bruising appearing under his face as the pair traded powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the verdict with scoring tallies of draw, close margin and wider gap, but the two athletes were relentless and full of heart, everything their inspiration Hatton embodied.
