British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker had his moments, landing some telling single shots in a contest where the styles failed to mesh, with constant grabbing spoiling the contest.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor commented in the ringside as he was met by boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, merited the decision but the British boxer endured the second loss of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Match
The fighter's advancement has slowed in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to overcommit in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the better rounds.
The crowd's patience wore thin amid regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific local rivalry contest to secure the continental championship.
The stadium fell silent for a extremely emotional tribute prior to the bout as 10 bells sounded in remembrance of local hero 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 concluding sound ended, the stillness transitioned to a chant of "there's only one Hatton".
The competition during the bout equaled the sentiment. Arthur was sent to the canvas by a significant blow flush on the face in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was injured soon, noticeable damage forming under his eye as the duo traded hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the result with official scores of 114-114, narrow lead and wider gap, but the two athletes were persistent and courageous, exactly what their idol Hatton stood for.
