UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a competitive ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a match where the approaches never quite clicked, with constant grabbing marring the contest.
The officials scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They succeed," the victor stated in the arena as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts believed the challenger, thirty-one, merited the win but the contender endured the second defeat of his 28-bout career.
Buatsi Earns Win in Dull Encounter
His career development has stalled in recent years through contract problems and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what feels like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective rounds.
The audience's tolerance decreased amid regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental championship.
The stadium grew hushed for a extremely emotional memorial before the fight as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a roar of "only one Ricky Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the early round, rising shakily to his feet on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up early, significant bruising appearing under his eye as the duo traded powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with scoring tallies of draw, close margin and wider gap, but each competitor were determined and brave, exactly what their inspiration Hatton embodied.
