UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a fight where the approaches didn't complement each other, with frequent clinching marring the fight.
Two judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They win," Buatsi commented in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the victory but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Claims Triumph in Slow Encounter
His career development has plateaued in recent years through contract problems and long spells of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to edge the more effective periods.
The crowd's patience decreased because of constant clinching, with the challenger going down several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to capture the continental belt.
The venue became quiet for a deeply poignant tribute before the fight as ten bells tolled in memory of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell disappeared, the silence became a roar of "there's only one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was knocked down by a left hook directly to the chin in the early round, rising shakily to his feet on the eight count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his orbital area as the duo traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
Arthur narrowly won the decision with judges scorecards of draw, 115-113 and wider gap, but both fighters were determined and courageous, exactly what their idol Hatton embodied.
