UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques didn't complement each other, with constant grabbing disrupting the fight.
Two judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the arena as he was greeted with disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought Parker, 31, merited the win but the British boxer suffered the second loss of his 28-bout professional journey.
The Champion Earns Win in Slow Affair
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with both competitors prepared to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a tight guard to edge the cleaner periods.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a outstanding all-Manchester contest to win the European light-heavyweight championship.
The venue became quiet for a extremely emotional homage ahead of the contest as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who died at age 46 in recently.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound ended, the silence gave way to a roar of "there's just one Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the early round, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up soon, noticeable damage developing under his orbital area as the fighters landed powerful shots.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the action seldom decreased over the full distance.
The victor barely secured the verdict with official scores of 114-114, 115-113 and clearer margin, but each competitor were persistent and full of heart, all the qualities their hero Hatton represented.
