UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the approaches never quite clicked, with constant grabbing spoiling the action.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They win," the victor commented in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, should have been awarded the victory but the contender suffered the second defeat of his 28-fight professional journey.
The Champion Secures Win in Uneventful Match
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers ready to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who utilized a solid defense to narrowly win the cleaner periods.
The crowd's patience diminished because of regular grabbing, with Parker falling to the floor various instances during clinches.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester battle to win the European light-heavyweight championship.
The venue grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the silence became a chant of "there's only one Ricky Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a left hook flush on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was injured early, significant bruising developing under his face as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were persistent and courageous, all the qualities their hero the legend represented.
