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 bronze medallist's work rate likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the techniques didn't complement each other, with frequent clinching spoiling the contest.
Two judges judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi commented in the ring as he was met by disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Secures Win in Slow Encounter
His career development has stalled in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under new promotional backing.
He opened tentatively, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a tight guard to edge the better stanzas.
The crowd's patience decreased during constant clinching, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester battle to secure the continental belt.
The venue fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell faded, the silence gave way to a shout of "only one Ricky Hatton".
The fighting within the ring equaled the feeling. Arthur was floored by a significant blow flush on the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured quickly, significant bruising developing under his face as the pair traded powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of draw, close margin and clearer margin, but each competitor were persistent and full of heart, exactly what their inspiration the legend embodied.
