UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the techniques never quite clicked, with constant grabbing spoiling the action.
The scoring judges had it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the ring as he was faced disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Various analysts believed Parker, 31, deserved the victory but the British boxer experienced the second setback of his 28-bout professional journey.
Buatsi Claims Win in Slow Affair
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a solid defense to edge the better stanzas.
The spectators' acceptance wore thin during frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry contest to win the European light heavyweight title.
The venue fell silent for a extremely emotional tribute before the fight as ten bells tolled in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime ended, the quiet gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout reflected the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, rising shakily to his feet on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured soon, significant bruising developing under his eye as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with official scores of draw, narrow lead and clearer margin, but the two athletes were persistent and full of heart, everything their idol Hatton stood for.
