UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a close ten-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the styles didn't complement each other, with excessive holding marring the action.
The officials scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," the victor said in the ring as he was met by disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
The Champion Earns Victory in Dull Encounter
His career development has plateaued in the past few years through contract problems and long spells of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to edge the better rounds.
The spectators' acceptance wore thin amid frequent holding, with Parker falling to the floor various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant homage prior to the bout as ten bells rang in remembrance of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "only one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was floored by a powerful punch squarely on the chin in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up soon, significant bruising forming under his eye as the pair traded powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of draw, close margin and 115-112, but each competitor were relentless and courageous, exactly what their idol Hatton stood for.
