British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a match where the approaches failed to mesh, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Some ringside observers thought Parker, 31, merited the win but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has slowed in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to narrowly win the cleaner rounds.
The audience's tolerance decreased during constant clinching, with Parker going down several times during clinches.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the European light heavyweight title.
The stadium became quiet for a profoundly moving memorial prior to the bout as 10 bells tolled in memory of local hero Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's passing. As the concluding sound disappeared, the silence became a roar of "there's just one Ricky Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the second round, standing uncertainly to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged early, noticeable damage forming under his face as the duo traded powerful shots.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
The victor barely secured the decision with judges scorecards of even score, close margin and clearer margin, but both fighters were determined and courageous, all the qualities their inspiration Hatton embodied.
