UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the styles never quite clicked, with excessive holding spoiling the contest.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They succeed," the victor said in the ring as he was met by disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, should have been awarded the victory but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He started carefully, with the two boxers ready to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to edge the more effective stanzas.
The crowd's patience diminished during constant clinching, with Parker falling to the floor several times during clinches.
Parker ended stronger, but many rounds were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a excellent local rivalry contest to win the continental championship.
The arena became quiet for a profoundly moving tribute prior to the bout as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the final chime faded, the silence became a shout of "only one Hatton".
The fighting during the bout equaled the emotion. Arthur was floored by a left hook directly to the chin in the early round, rising shakily to his position on the referee's count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling appearing under his eye as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the decision with judges scorecards of draw, 115-113 and wider gap, but both fighters were persistent and full of heart, everything their idol Hatton embodied.
