UK light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight 10-round bout at the boxing arena.
But Parker found success, landing some telling individual punches in a fight where the approaches failed to mesh, with excessive holding spoiling the fight.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best display but what defines champions? They succeed," the victor commented in the arena as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, deserved the win but the contender experienced the second setback of his 28-bout career.
The Champion Claims Win in Slow Match
The fighter's advancement has slowed in recent times through contract problems and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a solid defense to narrowly win the cleaner rounds.
The audience's tolerance decreased amid frequent holding, with Parker falling to the floor several times during clinches.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to win the European light heavyweight championship.
The venue fell silent for a deeply poignant tribute before the fight as ten bells sounded in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the city since the passing of Hatton. As the concluding sound disappeared, the quiet gave way to a shout of "only one Ricky Hatton".
The action during the bout equaled the feeling. The eventual winner was floored by a left hook flush on the face in the second round, rising shakily to his feet on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up soon, significant bruising developing under his orbital area as the pair traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
The victor barely secured the decision with scoring tallies of even score, narrow lead and 115-112, but the two athletes were relentless and courageous, everything their hero Hatton embodied.
