British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a match where the approaches didn't complement each other, with excessive holding spoiling the action.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my finest display but what do champions do? They prevail," the victor stated in the ring as he was faced disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, 31, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Earns Triumph in Dull Encounter
His career development has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors prepared to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who utilized a protective stance to edge the more effective rounds.
The crowd's patience decreased because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor 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 Tribute Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent local rivalry fight to secure the European light heavyweight title.
The venue grew hushed for a profoundly moving memorial ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a shout of "there's just one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was floored by a significant blow directly to the chin in the early round, rising shakily to his feet on the referee's count.
But the red-colored attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling forming under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor edged the result with scoring tallies of even score, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their idol the legend embodied.
