UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker found success, delivering effective single shots in a match where the approaches failed to mesh, with frequent clinching spoiling the contest.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," the victor commented in the ringside as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the victory but the contender experienced the second loss of his 28-fight professional journey.
The Champion Secures Win in Slow Encounter
Buatsi's progress has stalled in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who operated with a protective stance to edge the cleaner rounds.
The audience's tolerance wore thin amid constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Arthur defeated Rea by close verdict in a excellent local rivalry contest to secure the continental belt.
The venue became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound faded, the silence became a shout of "there's only one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was knocked down by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was marked up early, significant bruising appearing under his face as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the result with judges scorecards of 114-114, close margin and wider gap, but each competitor were relentless and courageous, everything their hero Hatton stood for.
