UK light heavyweight Joshua Buatsi made a winning yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques didn't complement each other, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They prevail," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the British boxer experienced the second loss of his 28-bout professional journey.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under recent support.
He began cautiously, with the two boxers willing to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective rounds.
The audience's tolerance diminished amid frequent holding, with the challenger going down several times during clinches.
Parker closed well, but several stanzas were difficult to score. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Arthur defeated Rea by split-decision in a excellent all-Manchester fight to win the European light heavyweight championship.
The venue became quiet for a profoundly moving tribute prior to the bout as ten bells tolled in remembrance of Manchester's own Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell faded, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The fighting during the bout reflected the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the early round, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising appearing under his eye as the fighters landed hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the decision with judges scorecards of 114-114, close margin and wider gap, but both fighters were determined and full of heart, everything their hero Hatton stood for.
