UK light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a match where the techniques never quite clicked, with frequent clinching marring the contest.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what do champions do? They win," Buatsi said in the ring as he was faced negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts believed Parker, thirty-one, merited the decision but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
The Champion Claims Triumph in Dull Match
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The spectators' acceptance decreased because of regular grabbing, with the challenger going down several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Arthur overcame Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to capture the European light-heavyweight belt.
The venue grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony tolled in remembrance of local hero Ricky 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 concluding sound disappeared, the stillness became a shout of "there's only one Hatton".
The fighting within the ring equaled the feeling. Arthur was knocked down by a powerful punch flush on the face in the initial stage, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged quickly, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor narrowly won the result with scoring tallies of even score, narrow lead and clearer margin, but both fighters were relentless and brave, all the qualities their hero Hatton embodied.
