UK light-heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a fight where the techniques failed to mesh, with frequent clinching marring the action.
Two judges judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi said in the ringside as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, should have been awarded the victory but the British boxer suffered the second defeat of his 28-bout career.
The Champion Earns Win in Slow Match
His career development has plateaued in recent times through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to edge the cleaner rounds.
The spectators' acceptance decreased during frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight championship.
The stadium fell silent for a deeply poignant homage before the fight as ten bells sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the face in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his orbital area as the pair traded hard punches.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, 115-113 and wider gap, but the two athletes were relentless and full of heart, everything their idol the legend embodied.
