UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round fight at the boxing arena.
But Parker had his moments, landing some telling single shots in a match where the techniques failed to mesh, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," the victor commented in the arena as he was met by negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Secures Win in Slow Match
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the more effective stanzas.
The audience's tolerance diminished because of regular grabbing, with the challenger going down several times during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester fight to win the European light heavyweight championship.
The venue became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound disappeared, the silence gave way to a roar of "only one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was knocked down by a significant blow flush on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured quickly, noticeable damage forming under his orbital area as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were relentless and courageous, everything their hero Hatton stood for.
