UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a fight where the styles didn't complement each other, with constant grabbing disrupting the fight.
Two judges scored it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They win," Buatsi commented in the ringside as he was faced boos from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Earns Win in Uneventful Match
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors prepared to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to barely take the better rounds.
The audience's tolerance decreased during regular grabbing, with the challenger going down several times during clinches.
Parker closed well, but many rounds were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Homage Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to win the European light-heavyweight championship.
The stadium fell silent for a extremely emotional homage before the fight as ten bells tolled in memory of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound ended, the quiet transitioned to a roar of "only one Ricky Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a powerful punch directly to the chin in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up quickly, significant bruising forming under his eye as the pair landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor barely secured the decision with judges scorecards of draw, narrow lead and wider gap, but the two athletes were determined and courageous, everything their idol the legend represented.
