British light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a competitive ten-round contest at the boxing arena.
But Parker had his moments, delivering effective single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the action.
Two judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, thirty-one, deserved the win but the British boxer experienced the second defeat of his 28-fight professional journey.
Buatsi Secures Win in Uneventful Encounter
The fighter's advancement has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter ready to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance wore thin because of regular grabbing, with the challenger falling to the floor various instances during clinches.
Parker ended stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight title.
The stadium grew hushed for a deeply poignant tribute ahead of the contest as a bell ceremony rang in memory of local hero Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the stillness gave way to a shout of "only one Hatton".
The competition during the bout reflected the emotion. Arthur was sent to the canvas by a powerful punch directly to the face in the second round, rising shakily to his feet on the eight count.
But the traditional red equipment of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged soon, heavy swelling forming under his face as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the result with scoring tallies of even score, narrow lead and clearer margin, but each competitor were relentless and full of heart, exactly what their idol Hatton stood for.
