UK light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker found success, landing some telling single shots in a match where the styles never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best performance but what do champions do? They succeed," the victor commented in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Several boxing experts believed the challenger, 31, should have been awarded the victory but the Derbyshire fighter endured the second loss of his 28-fight career.
Buatsi Secures Win in Dull Match
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a protective stance to barely take the more effective periods.
The spectators' acceptance wore thin because of constant clinching, with Parker going down several times during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the secondary fight, Arthur beat Rea by divided decision in a terrific all-Manchester fight to win the European light heavyweight belt.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who died 46 years old in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "there's only one Hatton".
The competition during the bout matched the sentiment. The eventual winner was floored by a significant blow squarely on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage appearing under his orbital area as the fighters landed punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the result with official scores of draw, narrow lead and wider gap, but the two athletes were relentless and full of heart, everything their inspiration the legend represented.
