UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a match where the styles failed to mesh, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi commented in the ringside as he was faced disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Claims Win in Uneventful Match
The fighter's advancement has slowed in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a tight guard to narrowly win the cleaner rounds.
The spectators' acceptance decreased amid constant clinching, with the challenger going down several times during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester contest to win the continental title.
The arena grew hushed for a extremely emotional tribute before the fight as ten bells rang in memory of local hero Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The action during the bout matched the emotion. Arthur was knocked down by a powerful punch flush on the jaw in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged early, noticeable damage forming under his eye as the fighters exchanged hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the result with judges scorecards of draw, close margin and clearer margin, but each competitor were determined and brave, everything their idol Hatton represented.
