British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a fight where the techniques didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but what defines champions? They succeed," the victor said in the arena as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the win but the contender endured the second defeat of his 28-match career.
The Champion Claims Win in Uneventful Match
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He began cautiously, with the two boxers ready to overcommit in a tentative initial rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to narrowly win the better periods.
The crowd's patience diminished because of frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Inspires Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester contest to secure the continental championship.
The arena grew hushed for a profoundly moving memorial before the fight as 10 bells rang in honor of local hero Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound faded, the silence gave way to a roar of "only one Hatton".
The competition within the ring matched the feeling. Arthur was floored by a left hook squarely on the jaw in the early round, getting up unsteadily to his position on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged quickly, significant bruising appearing under his eye as the fighters landed punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were determined and full of heart, everything their hero Hatton stood for.
