British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a match where the techniques didn't complement each other, with excessive holding marring the contest.
The officials had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They prevail," the victor said in the ring as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the win but the contender endured the second setback of his 28-match career.
The Champion Earns Triumph in Dull Match
The fighter's advancement has slowed in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers prepared to overcommit in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance decreased because of frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry battle to secure the European light-heavyweight belt.
The arena became quiet for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the quiet became a shout of "there's only one Ricky Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged early, significant bruising forming under his orbital area as the pair exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of even score, narrow lead and 115-112, but the two athletes were determined and brave, all the qualities their inspiration Hatton represented.
