British light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive 10-round contest at the boxing arena.
But Parker found success, delivering effective single shots in a match where the styles didn't complement each other, with excessive holding spoiling the contest.
Two judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best display but what defines champions? They prevail," Buatsi said in the ringside as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Various analysts believed Parker, 31, merited the win but the contender suffered the second loss of his 28-match professional journey.
Buatsi Secures Win in Dull Encounter
His career development has stalled in recent years through management issues and extended periods of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with the two boxers willing to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The spectators' acceptance diminished because of frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog closed well, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Contest
In the secondary fight, Arthur defeated Rea by split-decision in a terrific local rivalry fight to secure the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant homage ahead of the contest as 10 bells sounded in honor of local hero Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the concluding sound ended, the stillness transitioned to a chant of "only one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook flush on the chin in the early round, rising shakily to his position on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured quickly, heavy swelling developing under his eye as the fighters exchanged hard punches.
The challenger, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the intensity rarely dropped over the full distance.
The victor barely secured the verdict with judges scorecards of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, all the qualities their hero Hatton embodied.
