British light heavyweight Joshua Buatsi made a winning yet underwhelming start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a fight where the approaches never quite clicked, with excessive holding spoiling the action.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor stated in the ringside as he was faced boos from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, 31, deserved the win but the contender experienced the second loss of his 28-fight career.
The Champion Claims Triumph in Slow Encounter
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a solid defense to narrowly win the more effective periods.
The crowd's patience wore thin because of regular grabbing, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Arthur overcame Rea by divided decision in a terrific local rivalry contest to capture the European light heavyweight championship.
The stadium grew hushed for a deeply poignant memorial before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell disappeared, the silence became a roar of "only one Hatton".
The action during the bout equaled the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, rising shakily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was marked up soon, heavy swelling appearing under his face as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over the full distance.
Arthur edged the decision with official scores of 114-114, close margin and clearer margin, but each competitor were determined and brave, exactly what their hero Hatton embodied.
