British light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges scored it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," Buatsi commented in the arena as he was faced negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the decision but the contender experienced the second setback of his 28-match professional journey.
Buatsi Claims Win in Dull Affair
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who utilized a tight guard to edge the better rounds.
The audience's tolerance decreased during frequent holding, with Parker falling to the floor several times during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester fight to capture the continental championship.
The stadium grew hushed for a extremely emotional homage ahead of the contest as 10 bells rang in remembrance of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound ended, the silence gave way to a shout of "there's only one Hatton".
The fighting within the ring equaled the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was damaged soon, heavy swelling forming under his orbital area as the duo traded powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over the full distance.
Arthur narrowly won the result with scoring tallies of draw, close margin and 115-112, but each competitor were relentless and courageous, exactly what their hero the legend stood for.
