UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the approaches never quite clicked, with frequent clinching marring the contest.
The officials had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best performance but what defines champions? They win," the victor said in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-fight professional journey.
The Champion Claims Win in Uneventful Match
His career development has slowed in recent times through management issues and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective rounds.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Arthur beat Rea by divided decision in a outstanding all-Manchester fight to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving homage before the fight as a bell ceremony tolled in honor of the city's native Hatton, who died aged 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the final chime ended, the silence transitioned to a shout of "there's only one Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the chin in the initial stage, rising shakily to his feet on the referee's count.
But the traditional red attire of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up quickly, heavy swelling appearing under his face as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
The victor edged the decision with judges scorecards of even score, close margin and 115-112, but each competitor were determined and courageous, exactly what their hero Hatton stood for.
