British light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a competitive ten-round bout at the boxing arena.
But Parker found success, landing some telling clean blows in a fight where the approaches never quite clicked, with frequent clinching spoiling the fight.
The scoring judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They win," Buatsi said in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the decision but the British boxer experienced the second defeat of his 28-bout career.
Buatsi Earns Victory in Uneventful Match
His career development has stalled in recent times through management issues and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to edge the more effective periods.
The crowd's patience wore thin amid regular grabbing, with Parker falling to the floor several times during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the continental championship.
The venue fell silent for a extremely emotional tribute prior to the bout as ten bells sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the last bell disappeared, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red equipment of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was damaged soon, noticeable damage forming under his eye as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, 115-113 and clearer margin, but both fighters were determined and full of heart, all the qualities their inspiration the legend embodied.
