UK light-heavyweight Joshua Buatsi earned a win yet underwhelming start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a contest where the styles failed to mesh, with excessive holding marring the contest.
The officials had it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," the victor stated in the ringside as he was faced boos from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, 31, deserved the victory but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Dull 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 the British fight scene, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He started carefully, with the two boxers ready to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to barely take the better rounds.
The crowd's patience wore thin amid frequent holding, with the challenger going down various instances during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to capture the continental championship.
The venue grew hushed for a deeply poignant homage before the fight as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound ended, the quiet became a shout of "there's just one Ricky Hatton".
The competition within the ring matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the early round, rising shakily to his position on the count of eight.
But the traditional red attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was damaged early, heavy swelling appearing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of even score, narrow lead and wider gap, but the two athletes were determined and full of heart, all the qualities their idol Hatton embodied.
