UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round contest at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches never quite clicked, with frequent clinching marring the contest.
The scoring judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor commented in the ring as he was met by boos from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the win but the contender suffered the second setback of his 28-bout career.
The Champion Secures Victory in Uneventful Match
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 British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was making just his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a solid defense to edge the better stanzas.
The spectators' acceptance wore thin because of frequent holding, with the challenger slipping to the canvas several times during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a excellent local rivalry battle to secure the European light-heavyweight belt.
The stadium became quiet for a profoundly moving tribute prior to the bout as ten bells sounded in memory of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime ended, the quiet became a shout of "there's only one Hatton".
The action within the ring reflected the sentiment. Arthur was knocked down by a significant blow directly to the jaw in the early round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage developing under his face as the fighters landed hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
The victor barely secured the result with official scores of 114-114, 115-113 and clearer margin, but the two athletes were persistent and full of heart, exactly what their inspiration Hatton stood for.
