British light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close 10-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a contest where the techniques didn't complement each other, with frequent clinching spoiling the contest.
The scoring judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," the victor commented in the ringside as he was faced boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-match career.
The Champion Claims Victory in Slow Encounter
His career development has slowed in the past few years through management issues and long spells of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to barely take the more effective periods.
The spectators' acceptance decreased during frequent holding, with Parker falling to the floor several times during clinches.
Parker finished the stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding Manchester vs Manchester battle to capture the European light heavyweight title.
The arena grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in memory of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the location since Hatton's passing. As the last bell faded, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring reflected the sentiment. Arthur was sent to the canvas by a left hook directly to the face in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, noticeable damage appearing under his face as the duo traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with judges scorecards of even score, 115-113 and clearer margin, but the two athletes were persistent and courageous, all the qualities their hero the legend represented.
