UK light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close 10-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a contest where the techniques failed to mesh, with excessive holding spoiling the action.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what defines champions? They prevail," Buatsi stated in the ring as he was faced boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
Buatsi Earns Victory in Uneventful Encounter
The fighter's advancement has stalled in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to barely take the more effective rounds.
The audience's tolerance diminished amid regular grabbing, with the challenger going down several times during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding local rivalry battle to win the continental championship.
The stadium became quiet for a extremely emotional tribute ahead of the contest as 10 bells sounded in remembrance of the city's native Ricky Hatton, who died at age 46 in last month.
It was the first major televised fight night in the city since Hatton's death. As the last bell disappeared, the stillness became a chant of "only one Hatton".
The action within the ring reflected the feeling. Arthur was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage forming under his orbital area as the pair landed punishing blows.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor edged the verdict with judges scorecards of even score, close margin and 115-112, but the two athletes were relentless and brave, everything their hero Hatton stood for.
