British light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a match where the styles never quite clicked, with constant grabbing marring the contest.
The scoring judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best display but how do champions respond? They succeed," Buatsi said in the arena as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
Buatsi Earns Win in Dull Encounter
Buatsi's progress has slowed in recent times through contract problems and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what feels like a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to barely take the better stanzas.
The audience's tolerance diminished during frequent holding, with Parker falling to the floor several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to secure the continental belt.
The stadium fell silent for a profoundly moving memorial prior to the bout as a bell ceremony tolled in memory of the city's native Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the last bell disappeared, the silence transitioned to a roar of "there's just one Hatton".
The action during the bout equaled the sentiment. Arthur was floored by a left hook squarely on the face in the early round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling developing under his orbital area as the fighters exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the decision with official scores of even score, close margin and clearer margin, but the two athletes were persistent and courageous, all the qualities their idol the legend stood for.
