British light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round contest at the Manchester venue.
But the challenger landed significant shots, connecting with powerful single shots in a match where the approaches failed to mesh, with frequent clinching spoiling the fight.
The scoring judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," the victor commented in the arena as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, 31, merited the win but the Derbyshire fighter endured the second loss of his 28-match career.
The Champion Claims Victory in Uneventful Encounter
His career development has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who worked behind a solid defense to narrowly win the more effective periods.
The spectators' acceptance diminished during frequent holding, with Parker going down several times during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Inspires Manchester Derby Fight
In the co-main event, Arthur defeated Rea by close verdict in a outstanding local rivalry contest to secure the continental belt.
The stadium grew hushed for a deeply poignant memorial before the fight as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who died at age 46 in September.
It was the premier televised match card in the location since the passing of Hatton. As the final chime disappeared, the quiet gave way to a chant of "there's only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a significant blow squarely on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up quickly, heavy swelling forming under his orbital area as the fighters landed hard punches.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor edged the decision with judges scorecards of 114-114, close margin and 115-112, but each competitor were determined and courageous, all the qualities their idol the legend embodied.
