UK light heavyweight Joshua Buatsi made a winning yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round contest at the boxing arena.
But Parker found success, connecting with powerful single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what do champions do? They win," the victor stated in the ringside as he was faced negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the win but the British boxer endured the second setback of his 28-match career.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has plateaued in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a calculated early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to barely take the cleaner periods.
The audience's tolerance diminished amid constant clinching, with Parker going down several times during close exchanges.
Parker closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester battle to secure the continental title.
The venue fell silent for a extremely emotional tribute prior to the bout as a bell ceremony rang in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the last bell faded, the quiet gave way to a shout of "there's just one Hatton".
The competition within the ring reflected the feeling. Arthur was floored by a powerful punch flush on the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was marked up soon, heavy swelling forming under his orbital area as the pair traded punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the result with official scores of draw, narrow lead and 115-112, but both fighters were determined and brave, everything their hero Hatton stood for.
