UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a fight where the styles never quite clicked, with excessive holding spoiling the action.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," the victor said in the arena as he was greeted with boos from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers felt the challenger, 31, merited the victory but the Derbyshire fighter experienced the second loss of his 28-match career.
Buatsi Earns Triumph in Uneventful Match
Buatsi's progress has stalled in recent years through contract problems and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to realize his potential - perhaps in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who operated with a tight guard to narrowly win the more effective periods.
The audience's tolerance diminished amid frequent holding, with the challenger slipping to the canvas several times during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry battle to win the continental belt.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony tolled in remembrance of local hero Hatton, who died at age 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the last bell disappeared, the stillness transitioned to a shout of "only one Ricky Hatton".
The action during the bout matched the feeling. Arthur was floored by a powerful punch directly to the jaw in the initial stage, getting up unsteadily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage forming under his face as the duo traded powerful shots.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur barely secured the result with judges scorecards of even score, close margin and 115-112, but each competitor were persistent and courageous, everything their idol Hatton embodied.
