British light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with constant grabbing marring the action.
The scoring judges judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor commented in the arena as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt Parker, 31, deserved the win but the Derbyshire fighter endured the second setback of his 28-bout career.
Buatsi Claims Victory in Uneventful Encounter
His career development has stalled in recent years through management issues and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to barely take the more effective rounds.
The spectators' acceptance decreased during constant clinching, with the challenger going down various instances during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent local rivalry contest to capture the European light-heavyweight belt.
The stadium became quiet for a deeply poignant tribute prior to the bout as a bell ceremony rang in memory of local hero Hatton, who died 46 years old in recently.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime ended, the silence gave way to a shout of "there's only one Hatton".
The fighting during the bout matched the emotion. Arthur was floored by a powerful punch squarely on the face in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged quickly, heavy swelling forming under his orbital area as the duo landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of 114-114, close margin and 115-112, but the two athletes were persistent and brave, everything their hero Hatton stood for.
