UK light heavyweight Joshua Buatsi made a winning yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful single shots in a contest where the techniques didn't complement each other, with excessive holding disrupting the action.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but what defines champions? They prevail," the victor said in the ring as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the victory but the Derbyshire fighter experienced the second loss of his 28-bout career.
Buatsi Earns Victory in Uneventful Encounter
Buatsi's progress has stalled in recent times through contract problems and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He started carefully, with neither fighter ready to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to edge the better periods.
The crowd's patience diminished amid frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Motivates Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to win the continental belt.
The venue grew hushed for a extremely emotional homage prior to the bout as 10 bells sounded in honor of local hero Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound disappeared, the quiet gave way to a shout of "there's only one Ricky Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was knocked down by a powerful punch directly to the face in the second round, getting up unsteadily to his feet 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 injured soon, noticeable damage appearing under his face as the duo traded hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the result with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, all the qualities their hero Hatton embodied.
