UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight ten-round fight at the boxing arena.
But the challenger found success, landing some telling individual punches in a match where the styles failed to mesh, with excessive holding spoiling the action.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest display but how do champions respond? They win," Buatsi said in the ringside as he was greeted with disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, merited the decision but the British boxer endured the second loss of his 28-fight professional journey.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to overcommit in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience wore thin during frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester fight to capture the European light-heavyweight championship.
The stadium became quiet for a profoundly moving memorial prior to the bout as a bell ceremony tolled in honor of Manchester's own Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the stillness became a roar of "only one Hatton".
The competition within the ring equaled the sentiment. The eventual winner was floored by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, noticeable damage developing under his orbital area as the fighters landed punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur narrowly won the decision with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and brave, exactly what their inspiration the legend represented.
