UK light-heavyweight Joshua Buatsi made a winning yet disappointing 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 judges' verdict in a tight ten-round contest at the boxing arena.
But Parker had his moments, delivering effective single shots in a fight where the approaches never quite clicked, with frequent clinching spoiling the action.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They prevail," the victor commented in the arena as he was greeted with boos from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts thought Parker, 31, merited the victory but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
Buatsi Claims Win in Dull Affair
Buatsi's progress has stalled in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to overcommit in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to edge the better stanzas.
The crowd's patience decreased amid constant clinching, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the continental belt.
The stadium fell silent for a profoundly moving tribute prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Hatton, who passed away 46 years old in recently.
It was the first major televised match card in the location since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "only one Hatton".
The competition within the ring matched the emotion. Arthur was knocked down by a powerful punch directly to the face in the early round, rising shakily to his footing on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was injured early, significant bruising developing under his eye as the pair landed powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the result with judges scorecards of even score, 115-113 and wider gap, but the two athletes were determined and courageous, exactly what their idol Hatton embodied.
