British light heavyweight Joshua Buatsi earned a win yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round contest at the Manchester venue.
But Parker found success, delivering effective single shots in a contest where the styles never quite clicked, with frequent clinching disrupting the action.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They win," the victor said in the ring as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the victory but the British boxer experienced the second setback of his 28-fight career.
Buatsi Earns Victory in Slow Match
His career development has plateaued in recent times through management issues and long spells of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to edge the better rounds.
The audience's tolerance diminished because of regular grabbing, with Parker going down various instances during grappling.
Parker ended stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight championship.
The venue became quiet for a extremely emotional homage ahead of the contest as ten bells sounded in honor of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised boxing event in the city since Hatton's death. As the final chime faded, the stillness became a roar of "there's only one Ricky Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a left hook squarely on the jaw in the second round, standing uncertainly to his footing on the count of eight.
But the red-colored attire of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged early, heavy swelling developing under his orbital area as the fighters exchanged powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the result with official scores of 114-114, close margin and wider gap, but each competitor were persistent and courageous, exactly what their inspiration the legend stood for.
