British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a fight where the approaches failed to mesh, with constant grabbing disrupting the fight.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They win," Buatsi commented in the arena as he was faced boos from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, should have been awarded the win but the British boxer suffered the second defeat of his 28-fight career.
The Champion Secures Triumph in Uneventful Encounter
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to edge the cleaner rounds.
The crowd's patience decreased during constant clinching, with the challenger falling to the floor several times during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding local rivalry battle to capture the continental title.
The venue grew hushed for a profoundly moving memorial prior to the bout as ten bells sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the feeling. The eventual winner was floored by a significant blow directly to the face in the second round, standing uncertainly to his footing on the count of eight.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was injured early, noticeable damage developing under his orbital area as the fighters exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor narrowly won the verdict with judges scorecards of 114-114, 115-113 and 115-112, but each competitor were relentless and courageous, exactly what their idol Hatton represented.
