UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger had his moments, landing some telling single shots in a contest where the techniques didn't complement each other, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They prevail," the victor stated in the ringside as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
Buatsi Claims Victory in Slow Match
The fighter's advancement has plateaued in recent times through management issues and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who worked behind a protective stance to edge the cleaner rounds.
The spectators' acceptance decreased amid frequent holding, with Parker slipping to the canvas several times during clinches.
Parker closed well, but several stanzas were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels Manchester Derby Fight
In the co-main event, Arthur beat Rea by divided decision in a excellent local rivalry contest to capture the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial before the fight as 10 bells sounded in honor of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the location since Hatton's passing. As the concluding sound ended, the silence gave way to a roar of "there's only one Ricky Hatton".
The action during the bout equaled the sentiment. The eventual winner was floored by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored attire of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up soon, significant bruising forming under his eye as the pair exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with official scores of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, everything their hero Hatton embodied.
