UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with constant grabbing spoiling the fight.
The officials had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest display but how do champions respond? They win," Buatsi commented in the arena as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Secures Victory in Slow Encounter
His career development has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to fulfil that early promise - 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 early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who worked behind a protective stance to edge the cleaner periods.
The audience's tolerance decreased because of frequent holding, with the challenger going down several times during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester fight to capture the continental title.
The arena became quiet for a extremely emotional tribute prior to the bout as a bell ceremony tolled in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime ended, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes equaled the emotion. Arthur was floored by a significant blow directly to the jaw in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was injured early, noticeable damage appearing under his face as the pair landed hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor edged the result with official scores of even score, 115-113 and clearer margin, but the two athletes were relentless and brave, exactly what their inspiration the legend embodied.
