British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a close ten-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a fight where the styles failed to mesh, with frequent clinching disrupting the action.
Two judges had it 96-94 to the winner, with the third a 95-95.
"It wasn't my best display but how do champions respond? They win," the victor said in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, 31, deserved the decision but the British boxer experienced the second loss of his 28-match career.
Buatsi Secures Triumph in Slow Match
His career development has plateaued in recent times through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He started carefully, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who operated with a tight guard to narrowly win the more effective periods.
The spectators' acceptance diminished during regular grabbing, with the challenger going down various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a terrific local rivalry fight to capture the European light-heavyweight title.
The arena fell silent for a profoundly moving tribute prior to the bout as a bell ceremony tolled in honor of the city's native Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a chant of "there's just one Hatton".
The action during the bout equaled the feeling. Arthur was knocked down by a powerful punch flush on the jaw in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, noticeable damage appearing under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur edged the result with scoring tallies of 114-114, 115-113 and 115-112, but each competitor were determined and courageous, exactly what their idol the legend stood for.
