UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the approaches never quite clicked, with excessive holding marring the contest.
The officials scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best performance but how do champions respond? They win," Buatsi stated in the ringside as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, deserved the victory but the British boxer experienced the second setback of his 28-match professional journey.
The Champion Earns Win in Slow Encounter
The fighter's advancement has stalled in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The crowd's patience wore thin because of regular grabbing, with the challenger going down several times during grappling.
Parker closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Fuels Local Rivalry Contest
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester battle to secure the continental title.
The venue fell silent for a extremely emotional homage before the fight as 10 bells rang in memory of local hero Ricky Hatton, who died at age 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the final chime disappeared, the silence gave way to a roar of "there's just one Hatton".
The competition inside the ropes equaled the feeling. Arthur was floored by a left hook squarely on the chin in the second round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up early, significant bruising appearing under his face as the pair exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur narrowly won the verdict with official scores of draw, close margin and clearer margin, but each competitor were relentless and brave, everything their inspiration the legend embodied.
