British light-heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive ten-round bout at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Slow Affair
The fighter's advancement has stalled in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a protective stance to barely take the more effective rounds.
The spectators' acceptance wore thin during constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
Parker closed well, but multiple periods were challenging to assess. However, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester contest to capture the European light-heavyweight belt.
The stadium became quiet for a profoundly moving memorial before the fight as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the city since the passing of Hatton. As the last bell faded, the quiet became a shout of "there's just one Hatton".
The fighting during the bout matched the feeling. The eventual winner was floored by a left hook directly to the chin in the initial stage, standing uncertainly to his position on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was marked up quickly, heavy swelling appearing under his orbital area as the pair exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
The victor edged the result with official scores of 114-114, 115-113 and wider gap, but each competitor were relentless and courageous, everything their idol the legend embodied.
