UK light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the styles didn't complement each other, with constant grabbing marring the action.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi stated in the ringside as he was met by disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts thought Parker, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
The Champion Claims Triumph in Uneventful Affair
Buatsi's progress has plateaued in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the cleaner periods.
The audience's tolerance wore thin during constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but many rounds were hard to judge. However, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Arthur defeated Rea by split-decision in a terrific all-Manchester fight to secure the European light heavyweight championship.
The venue became quiet for a profoundly moving memorial prior to the bout as a bell ceremony sounded in remembrance of local hero Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell faded, the stillness gave way to a chant of "there's only one Ricky Hatton".
The competition during the bout matched the emotion. Arthur was knocked down by a powerful punch flush on the face in the early round, standing uncertainly to his position on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, noticeable damage developing under his eye as the duo landed punishing blows.
Rea, 27, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the decision with official scores of draw, close margin and wider gap, but each competitor were relentless and brave, everything their inspiration Hatton stood for.
